drm/i915/sdov: switch IS_SDVOB to a flag

With valleyview we'll have these at yet another address, so keeping
track of this with an ever-growing list of registers will get ugly.

This way intel_sdvo.c is fully independent of the base address of the
output ports display register blocks.

While at it, do 2 closely related cleanups:
- use SDVO_NAME some more
- change the sdvo_reg variables to uint32_t like other registers.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>