drm/i915/guc: Don't call switch_to_kernel_context with GuC submission

Calling switch_to_kernel_context isn't needed if the engine PM reference
is taken while all user contexts are pinned as if don't have PM ref that
guarantees that all user contexts scheduling is disabled. By not calling
switch_to_kernel_context we save on issuing a request to the engine.

v2:
 (Daniel Vetter)
  - Add FIXME comment about pushing switch_to_kernel_context to backend
v3:
 (John Harrison)
  - Update commit message
  - Fix workding comment

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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/20211014172005.27155-5-matthew.brost@intel.com