drm/i915/vga: switch to use VGA definitions from video/vga.h

The video/vga.h has macros for the VGA registers. Switch to use them.

v2: Use direct 0x01 instead of the confusing VGA_SEQ_CLOCK_MODE (Ville)

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/20220202112509.1886660-1-jani.nikula@intel.com