drm/i915: remove VGA register definitions

The only user of the VGA registers has switched to using the definitions
in linux/vga.h, so these have become redundant. Remove them.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220202112509.1886660-2-jani.nikula@intel.com