drm/i915: fix compilation warnings on maybe uninitialized pointers

Two warnings are produced by gcc (tested with gcc 6.2.1):

drivers/gpu/drm/i915/intel_csr.c: In function ‘csr_load_work_fn’:
drivers/gpu/drm/i915/intel_csr.c:400:5: error: ‘fw’ is used
uninitialized in this function [-Werror=uninitialized]
  if (fw)
       ^
and

In file included from drivers/gpu/drm/i915/i915_drv.h:47:0,
                 from drivers/gpu/drm/i915/intel_guc_loader.c:30:
drivers/gpu/drm/i915/intel_guc_loader.c: In function ‘intel_guc_init’:
./include/drm/drmP.h:228:2: error: ‘fw’ may be used uninitialized in this
function  -Werror=maybe-uninitialized]
  drm_printk(KERN_DEBUG, DRM_UT_DRIVER, fmt, ##__VA_ARGS__)
  ^~~~~~~~~~
drivers/gpu/drm/i915/intel_guc_loader.c:595:25: note: ‘fw’ was declared here
  const struct firmware *fw;
                         ^~

When CONFIG_DRM_I915_WERROR is set, those warnings break the build.

Initializing fw pointer to NULL in both cases removes the warnings.
Signed-off-by: NJérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128234319.20800-1-jeremy.lefaure@lse.epita.fr

drivers/gpu/drm/i915/intel_csr.c

@@ -389,7 +389,7 @@ static void csr_load_work_fn(struct work_struct *work)
 {
 	struct drm_i915_private *dev_priv;
 	struct intel_csr *csr;
-	const struct firmware *fw;
+	const struct firmware *fw = NULL;
 	int ret;
 
 	dev_priv = container_of(work, typeof(*dev_priv), csr.work);

drivers/gpu/drm/i915/intel_guc_loader.c

@@ -592,7 +592,7 @@ static void guc_fw_fetch(struct drm_device *dev, struct intel_guc_fw *guc_fw)
 {
 	struct pci_dev *pdev = dev->pdev;
 	struct drm_i915_gem_object *obj;
-	const struct firmware *fw;
+	const struct firmware *fw = NULL;
 	struct guc_css_header *css;
 	size_t size;
 	int err;