drm/i915/guc: enable only the user interrupt when using GuC submission

In GuC submission mode the CS is owned by the GuC FW, so all CS status
interrupts are handled by it. We only need the user interrupt as that
signals request completion.

Since we're now starting the engines directly in GuC submission mode
when selected, we can stop switching back and forth between the
execlists and the GuC programming and select directly the correct
interrupt mask.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603051630.2635-4-matthew.brost@intel.com