drm/i915: Fix kzalloc() smatch warnings in get_initial_plane_config()

Smatch doesn't like:

  struct drm_framebuffer *fb;
  fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);

and warns with:

  warn: struct type mismatch 'drm_framebuffer vs intel_framebuffer'

This implicit cast was correct as struct intel_framebuffer has struct
drm_framebuffer as its first member, but in case someone want to reorder
the fields for some reason, it's slightly safer to access the underlying
drm_framebuffer through intel_fb->base.

Also, having fewer static analysis warnings is a worthy goal.

Cc: kbuild@01.org
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6584,13 +6584,16 @@ i9xx_get_initial_plane_config(struct intel_crtc *crtc,
 	int fourcc, pixel_format;
 	int aligned_height;
 	struct drm_framebuffer *fb;
+	struct intel_framebuffer *intel_fb;
 
-	fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
-	if (!fb) {
+	intel_fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
+	if (!intel_fb) {
 		DRM_DEBUG_KMS("failed to alloc fb\n");
 		return;
 	}
 
+	fb = &intel_fb->base;
+
 	val = I915_READ(DSPCNTR(plane));
 
 	if (INTEL_INFO(dev)->gen >= 4)
@@ -7613,13 +7616,16 @@ skylake_get_initial_plane_config(struct intel_crtc *crtc,
 	int fourcc, pixel_format;
 	int aligned_height;
 	struct drm_framebuffer *fb;
+	struct intel_framebuffer *intel_fb;
 
-	fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
-	if (!fb) {
+	intel_fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
+	if (!intel_fb) {
 		DRM_DEBUG_KMS("failed to alloc fb\n");
 		return;
 	}
 
+	fb = &intel_fb->base;
+
 	val = I915_READ(PLANE_CTL(pipe, 0));
 	if (val & PLANE_CTL_TILED_MASK)
 		plane_config->tiling = I915_TILING_X;
@@ -7706,13 +7712,16 @@ ironlake_get_initial_plane_config(struct intel_crtc *crtc,
 	int fourcc, pixel_format;
 	int aligned_height;
 	struct drm_framebuffer *fb;
+	struct intel_framebuffer *intel_fb;
 
-	fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
-	if (!fb) {
+	intel_fb = kzalloc(sizeof(struct intel_framebuffer), GFP_KERNEL);
+	if (!intel_fb) {
 		DRM_DEBUG_KMS("failed to alloc fb\n");
 		return;
 	}
 
+	fb = &intel_fb->base;
+
 	val = I915_READ(DSPCNTR(pipe));
 
 	if (INTEL_INFO(dev)->gen >= 4)