drm/i915/bios: assume eDP is present on port A when there is no VBT

We rely on VBT DDI port info for eDP detection on GEN9 platforms and
above. This breaks GEN9 platforms which don't have VBT because port A
eDP now defaults to false. Fix this by defaulting to true when VBT is
missing.

Fixes: a98d9c1d

 ("drm/i915/ddi: Rely on VBT DDI port info for eDP detection")
Signed-off-by: Thomas Preston <thomas.preston@codethink.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306200618.17405-1-thomas.preston@codethink.co.uk
(cherry picked from commit 2131bc0c

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>