drm/xe: Check skip_guc_pc before disabling gucrc

Also, use the new C6 helper instead of duplicating that code.

v2: Check skip flag at the beginning of the function (Rodrigo)

Fixes: 975e4a37 ("drm/xe: Manually setup C6 when skip_guc_pc is set")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>