drm/xe/guc_pc: Add _locked variant for min/max freq

drm/xe/guc_pc: Add _locked variant for min/max freq

There are places in which the getters/setters are called one after the
other causing a multiple lock()/unlock(). These are not currently a
problem since they are all happening from the same thread, but there's a
race possibility as calls are added outside of the early init when the
max/min and stashed values need to be correlated.

Add the _locked() variants to prepare for that.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250618-wa-22019338487-v5-1-b888388477f2@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>