doc: drm: Fix mis-spelling of i915_guc_submission includes

In commit
 d1675198: drm/i915: Integrate GuC-based command submission

the drm.tmpl include lines reference the intel_guc_submission.c but the
patch adds the file i915_guc_submission.c. drm.tmpl fails to build with:
 docproc: .//drivers/gpu/drm/i915/intel_guc_submission.c:
  No such file or directory

Change the file reference to the actual file.

Signed-off-by: Graham Whaley <graham.whaley@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>