drm/i915: Reset GPU immediately if submission is disabled

If submission is disabled by the backend for any reason, reset the GPU
immediately in the heartbeat code as the backend can't be reenabled
until the GPU is reset.

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/20210727002348.97202-10-matthew.brost@intel.com