drm/i915: Rip out SUPPORTS_EDP

It only controls the setting of the vbt.edp_support variable, which in
turn only controls one debug output plus can also force-disable the
lvds output.

Since the value only restricted this logic to mobile ilk there's the
slight risk that this will break lvds on desktop ilk or on snb/ivb
platforms. But with the vbt it's better when we know what's going on
here, so let's rip it out and see what happens.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>