i915, debugfs: Fix uninitialized warning

gcc complains that:

drivers/gpu/drm/i915/i915_debugfs.c: In function ‘display_crc_ctl_write’:
drivers/gpu/drm/i915/i915_debugfs.c:2393:2: warning: ‘val’ may be used uninitialized in this function [-Wuninitialized]
drivers/gpu/drm/i915/i915_debugfs.c:2350:6: note: ‘val’ was declared here

but it can't see that we're going to use val only in the success case.
So shut it up.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>