doc: Fix build of documentation after i915 file rename

Commit 70a2b431 ("drm/i915/gt: Rename lrc.c to
execlists_submission.c") renamed intel_lrc.c to
intel_execlists_submission.c but forgot to update i915.rst.

Fixes: 70a2b431 ("drm/i915/gt: Rename lrc.c to execlists_submission.c")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201214185440.243537-1-jose.souza@intel.com