drm/i915/reg: separate VLV_DSPCLK_GATE_D from DSPCLK_GATE_D

drm/i915/reg: separate VLV_DSPCLK_GATE_D from DSPCLK_GATE_D

All the places that use DSPCLK_GATE_D are specific to certain platforms,
and the parametrization of it to support VLV/CHV MMIO display base isn't
really buying us anything. Add a separate macro for VLV_DSPCLK_GATE_D
and use it.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/ac16d9d5192595944bf9bcf70aa721b504bc90c0.1754499175.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>