drm/i915/guc: Do not conflate lrc_desc with GuC id for registration

The LRC descriptor pool is going away. So, stop using it as a check for
context registration, use the GuC id instead (being the thing that
actually gets registered with the GuC).

Also, rename the set/clear/query helper functions for context id
mappings to better reflect their purpose and to differentiate from
other registration related helper functions.

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/20220302003357.4188363-2-John.C.Harrison@Intel.com