drm/i915: Fix coding style on DPLL*_ENABLE defines

Abide by the rules in the top of the header: 2 spaces for bitfield,
prefix offsets with underscore and prefer the use of REG_BIT().

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120193457.3295977-3-lucas.demarchi@intel.com