drm/xe: Use spinlock in forcewake instead of mutex

In PMU we need to access certain registers which fall under GT power
domain for which we need to take forcewake. But as PMU being an atomic
context can't expect to have any sleeping calls.

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Aravind Iddamsetty <aravind.iddamsetty@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>