drm/xe: Remove mem_access from guc_pc calls

We are now protected by init, sysfs, or removal and don't
need these mem_access protections around GuC_PC anymore.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222163937.138342-6-rodrigo.vivi@intel.com