drm/xe/guc: Prevent use of uninitialized mutex

When skip_guc_pc is set and/or this is for a VF.

Fixes: 3b1592fb ("drm/xe/lnl: Apply Wa_22019338487")
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240701231529.2582452-1-vinay.belgaumkar@intel.com


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>