drm/xe/guc: Add pc_to_ct() helper

We are converting xe_guc_pc to xe_guc_ct few times already.
Add simple helper function to avoid code duplication.
While at it, simplify other helper functions and fix order
of local variables to match the guideline.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240609181931.1724-3-michal.wajdeczko@intel.com