drm/i915/reg: use REG_BIT and friends to define DP registers

Define the DP register contents using the REG_BIT, REG_GENMASK,
etc. macros. Ditch the unhelpful comments. Rename eDP related register
content macros to have EDP_ prefix.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20250423100213.720585-1-jani.nikula@intel.com


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