drm/i915/reg: split out vlv_dsi_regs.h and vlv_dsi_pll_regs.h

The VLV (including CHV, BXT, and GLK) DSI registers have fairly isolated
usage. Split the register macros to separated files.

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217224023.3994777-3-jani.nikula@intel.com