drm/i915: Discard BIOS framebuffers too small to accommodate chosen mode

If the inherited BIOS framebuffer is smaller than the mode selected for
fbdev, then if we continue to use it then we cause display corruption as
we do not setup the panel fitter to upscale.

Regression from commit d978ef14
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Mar 7 08:57:51 2014 -0800

    drm/i915: Wrap the preallocated BIOS framebuffer and preserve for KMS fbcon v12

v2: Add a debug message to track the discard of the BIOS fb.
v3: Ville pointed out the difference between ref/unref
Reported-by: NKnut Petersen <Knut_Petersen@t-online.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77767Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_fbdev.c

@@ -132,6 +132,16 @@ static int intelfb_create(struct drm_fb_helper *helper,
 
 	mutex_lock(&dev->struct_mutex);
 
+	if (intel_fb &&
+	    (sizes->fb_width > intel_fb->base.width ||
+	     sizes->fb_height > intel_fb->base.height)) {
+		DRM_DEBUG_KMS("BIOS fb too small (%dx%d), we require (%dx%d),"
+			      " releasing it\n",
+			      intel_fb->base.width, intel_fb->base.height,
+			      sizes->fb_width, sizes->fb_height);
+		drm_framebuffer_unreference(&intel_fb->base);
+		intel_fb = ifbdev->fb = NULL;
+	}
 	if (!intel_fb || WARN_ON(!intel_fb->obj)) {
 		DRM_DEBUG_KMS("no BIOS fb, allocating a new one\n");
 		ret = intelfb_alloc(helper, sizes);