drm/i915/guc: Implement GuC priority management

Implement a simple static mapping algorithm of the i915 priority levels
(int, -1k to 1k exposed to user) to the 4 GuC levels. Mapping is as
follows:

i915 level < 0          -> GuC low level     (3)
i915 level == 0         -> GuC normal level  (2)
i915 level < INT_MAX    -> GuC high level    (1)
i915 level == INT_MAX   -> GuC highest level (0)

We believe this mapping should cover the UMD use cases (3 distinct user
levels + 1 kernel level).

In addition to static mapping, a simple counter system is attached to
each context tracking the number of requests inflight on the context at
each level. This is needed as the GuC levels are per context while in
the i915 levels are per request.

v2:
 (Daniele)
  - Add BUILD_BUG_ON to enforce ordering of priority levels
  - Add missing lockdep to guc_prio_fini
  - Check for return before setting context registered flag
  - Map DISPLAY priority or higher to highest guc prio
  - Update comment for guc_prio

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-33-matthew.brost@intel.com