drm/i915/guc: Better name for context id limit

The LRC descriptor pool is going away. So, stop using it as the limit
for how many context ids are available. Instead, size the pool
according to the number of contexts allowed. Note that this is just a
naming change, the actual limit is identical in value.

While at it, also update a kzalloc(sizeof()*count) to be a
kcalloc(count,size).

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/20220302003357.4188363-4-John.C.Harrison@Intel.com