drm/i915: Convert PLL flags to booleans

No real reason why the PLL flags need to be a bitmask. Switch
to booleans to make the code simpler.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240123093137.9133-3-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>