drm/i915/display: abstract all vgaarb access to intel_vga.[ch]

Split out the code related to vga client and vgaarb all over the place
into new intel_vga.[ch]. No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191001152506.7854-1-jani.nikula@intel.com