drm/i915: Fix the VLV/CHV DSI panel orientation hw readout

drm/i915: Fix the VLV/CHV DSI panel orientation hw readout

Let's make sure the DSI port is actually on before we go
poking at the plane register to determine which way
it's rotated. Otherwise we could be looking at a plane
that is feeding a HDMI port for instance.

And in order to read the plane register we need the power
well to be on. Make sure that is indeed the case. We'll
also make sure the plane is actually enabled before we
trust the rotation bit to tell us the truth.

v2: s/intel_dsi/vlv_dsi/

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181022141953.3889-1-ville.syrjala@linux.intel.com
Tested-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc