drm/i915/guc/slpc: Remove BUG_ON in guc_submission_disable

The assumption when it was added was that GT would not be
holding any gt_pm references. However, uc_init is called
from gt_init_hw, which holds a forcewake ref. If SLPC
enable fails, we will still be holding this ref, which will
result in the BUG_ON.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730202119.23810-7-vinay.belgaumkar@intel.com