drm/i915/gt: Rename lrc.c to execlists_submission.c

We want to separate the utility functions for controlling the logical
ring context from the execlists submission mechanism (which is an
overgrown scheduler).

This is similar to Daniele's work to split up the files, but being
selfish I wanted to base it after my own changes to intel_lrc.c petered
out.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209233618.4287-2-chris@chris-wilson.co.uk