drm/i915: Expose one LRC function for GuC submission mode

drm/i915: Expose one LRC function for GuC submission mode

GuC submission is basically execlist submission, but with the GuC
handling the actual writes to the ELSP and the resulting context
switch interrupts.  So to describe a context for submission via
the GuC, we need one of the same functions used in execlist mode.
This commit exposes one such function, changing its name to better
describe what it does (it's related to logical ring contexts rather
than to execlists per se).

v2:
    Replaces previous "drm/i915: Move execlists defines from .c to .h"

v3:
    Incorporates a change to one of the functions exposed here that was
        previously part of an internal patch, but which was omitted from
        the version recently committed to drm-intel-nightly:
	    4d00591 drm/i915/lrc: Update PDPx registers with lri commands
        So we reinstate this change here.

v4:
    Drop v3 change, update function parameters due to collision with
        bc3f567 drm/i915: Convert execlists_ctx_descriptor() for requests

v5:
    Don't expose execlists_update_context() after all. The current
        version is no longer compatible with GuC submission; trying to
        share the execlist version of this function results in both GuC
        and CPU updating TAIL in the context image, with bad results when
        they get out of step. The GuC submission path now has its own
        private version that just updates the ringbuffer start address,
        and not TAIL or PDPx.

v6:
    Rebased

Issue: VIZ-4884
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>