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 <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Reviewed-by: Lucas De Marchi <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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