drm/i915/guc: Split guc_lrc_desc_pin apart

The LRC descriptor pool is going away. Further, the function that was
populating it was also doing a bunch of logic about the context
registration sequence. So, split that code apart into separate state
setup and try to register functions. Note that some of those 'try to
register' code paths actually undo the state setup and leave it to be
redone again later (with potentially different values). This is
inefficient. The next patch will correct this.

Also, move a comment about ignoring return values to the place where
the return values are actually ignored.

v2: Move some more splitting from a later patch (and do it correctly).

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302003357.4188363-5-John.C.Harrison@Intel.com