drm/i915: fix compilation warnings on maybe uninitialized pointers

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: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128234319.20800-1-jeremy.lefaure@lse.epita.fr