drm/i915: Check VBT for eDP ports on VLV

VLV can have eDP on either port B or C, or even both. Based on the
VBT spec, intel_dpd_is_edp() should work on VLV too, assuming we
check the correct ports.

So instead of hardcoding port D, rename the function to
intel_dp_is_edp() and pass the port as a parameter, and use it
on VLV ports B and C.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71051


Tested-by: Robert Hooker <robert.hooker@canonical.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Wrestle the patch to apply and compile properly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>