drm/xe: Manually setup C6 when skip_guc_pc is set

Skip the init/start/stop GuC PC functions and toggle C6 using
register writes instead. Also request max possible frequency
as dynamic freq management is disabled.

v2: Fix compile warning

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>