drm/i915/guc: Drop pin count check trick between sched_disable and re-pin

Drop pin count check trick between a sched_disable and re-pin, now rely
on the lock and counter of the number of committed requests to determine
if scheduling should be disabled on the context.

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-20-matthew.brost@intel.com