drm/i915/dp: Use BDB_GENERAL_FEATURES VBT block info for builtin panel-orientation

Some devices with a builtin panel have the panel mounted upside down,
this is indicated by the rotate_180 bit in the BDB_GENERAL_FEATURES VBT
block.

We store this info in dev_priv->vbt.orientation, use this to set the
connector's orientation property so that fbcon and userspace will show
the image the right way up on devices with an upside-down mounted panel.

This fixes the image being upside-down on a Teclast X89 tablet.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228114110.187792-3-hdegoede@redhat.com