drm/i915: s/is_sdvob/enum port/

Replace the is_sdvob bool and some sdvo_reg checks with enum port. This
makes the SDVO code look more modern, and gets rid of explicit register
offset checks in the code which will hamper register type checking.

v2: Add assert_sdvo_port_valid() (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1446838199-3666-1-git-send-email-ville.syrjala@linux.intel.com