drm/i915: implement workaround for VTOTAL when using TRANSCODER_EDP

See the documentation for the DDI_FUNC_CTL register, EDP Input Select
bits: when the EDP input selection is B, the VTOTAL_B must be
programmed with the VTOTAL_EDP value, same thing for selection C.

V2: Use I915_READ as suggested by Daniel Vetter.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>