drm/i915/guc: Refactor the concept "GuC context descriptor" into "GuC stage descriptor"

A GuC context and a HW context are in no way related, so the name "GuC context descriptor"
is very unfortunate, because a new reader of the code gets overwhelmed very quickly with
a lot of things called "context" that refer to different things. We can improve legibility
a lot by simply renaming a few objects in the GuC code.

v2:
  - Rebased
  - s/ctx_desc_pool/stage_desc_pool
  - Move some explanations to the definition of the guc_stage_desc struct (Chris)

v3:
  - Calculate gemsize with less intermediate steps (Joonas)
  - Use BIT() macro (Joonas)

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>