drm/xe/debugfs: grab mem_access around forcewake

We need keep the device awake when performing any kind of mmio operation.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/279


Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>