drm/i915: Introduce i915_vgacntrl_reg()

The VGACNTRL register has moved around between different platforms.
To handle the differences add i915_vgacntrl_reg() which returns the
correct offset for the VGACNTRL register.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>