drm/i915/dp: use single point of truth for PPS divisor register

Set pp_div field of struct pps_registers to INVALID_MMIO_REG when the
register isn't there, and use i915_mmio_reg_valid() instead of repeating
the condition all over the place.

Use INVALID_MMIO_REG explicitly for documentation purposes, even if the
value is unchanged from 0.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305135215.29862-2-jani.nikula@intel.com