drm/xe: Grab mem_access when disabling C6 on skip_guc_pc platforms

drm/xe: Grab mem_access when disabling C6 on skip_guc_pc platforms

If skip_guc_pc is set for a platform, C6 is disabled directly without
acquiring a mem_access reference, triggering an assertion inside
xe_gt_idle_disable_c6.

Fixes: 975e4a3795d4 ("drm/xe: Manually setup C6 when skip_guc_pc is set")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240126220613.865939-2-matthew.d.roper@intel.com