drm/i915/gt: Move move context layout registers and offsets to lrc_reg.h

Cleanup intel_lrc.h by moving some of the residual common register
definitions into intel_lrc_reg.h, prior to rebranding and splitting off
the submission backends.

v2: keep the SCHEDULE enum in the old file, since it is specific to the
gvt usage of the execlists submission backend (John)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> #v2
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209233618.4287-1-chris@chris-wilson.co.uk