drm/i915: Turn on panel power before doing aux transfers

On VLV/CHV the panel power sequencer may need to be "kicked" a bit to
lock onto the new port, and that needs to happen before any aux
transfers are attempted if we want the aux transfers to actaully
succeed. So turn on panel power (part of the "kick") before aux
transfers (DPMS_ON + link training).

This also matches the documented modeset sequence better for pch
platforms. The documentation doesn't explicitly state anything about the
DPMS or link training DPCD writes, but the panel power on step is
always listed before link training is mentioned.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=70117
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>