documentation/gpu/i915: fix docs build error after file rename

Fix documentation build errors after intel_guc_loader.c was
renamed to intel_guc_fw.c.

Error: Cannot open file ../drivers/gpu/drm/i915/intel_guc_loader.c
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -function GuC-specific firmware loader ../drivers/gpu/drm/i915/intel_guc_loader.c' failed with return code 1
Error: Cannot open file ../drivers/gpu/drm/i915/intel_guc_loader.c
Error: Cannot open file ../drivers/gpu/drm/i915/intel_guc_loader.c
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -internal ../drivers/gpu/drm/i915/intel_guc_loader.c' failed with return code 2

Fixes: e8668bbc ("drm/i915/guc: Rename intel_guc_loader.c to intel_guc_fw.c")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1b214f53-47f5-bef3-f58e-8136de5678ed@infradead.org


(cherry picked from commit 006c2332)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>