drm/i915/guc: Take engine PM when a context is pinned with GuC submission

Taking a PM reference to prevent intel_gt_wait_for_idle from short
circuiting while any user context has scheduling enabled. Returning GT
idle when it is not can cause all sorts of issues throughout the stack.

v2:
 (Daniel Vetter)
  - Add might_lock annotations to pin / unpin function
v3:
 (CI)
  - Drop intel_engine_pm_might_put from unpin path as an async put is
    used
v4:
 (John Harrison)
  - Make intel_engine_pm_might_get/put work with GuC virtual engines
  - Update commit message
v5:
  - Update commit message again

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014172005.27155-4-matthew.brost@intel.com