drm/i915/perf: use the lrc_desc to get the ctx hw id in gen8-10

drm/i915/perf: use the lrc_desc to get the ctx hw id in gen8-10

The upper 32 bits of the lrc_desc (bits 52-32 to be precise) are the
context hw id in GEN8-10, so use them and have one less thing to
maintain in the unlikely case we change the descriptor sw fields.

v2: If we use the lrc_desc, we must apply the ctx_id_mask too (Lionel)

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180604233250.609-1-michel.thierry@intel.com