Try to detect the max TMDS clock limit for the DP++ adaptor (if any)
and take it into account when checking the port clock.

Note that as with the sink (HDMI vs. DVI) TMDS clock limit we'll ignore
the adaptor TMDS clock limit in the modeset path, in case users are
already "overclocking" their TMDS links. One subtle change here is that
we'll have to respect the adaptor TMDS clock limit when we decide whether
to do 12bpc or 8bpc, otherwise we might end up picking 12bpc and
accidentally driving the TMDS link out of spec even when the user chose
a mode that fits wihting the limits at 8bpc. This means you can't
"overclock" your DP++ dongle at 12bpc anymore, but you can continue to
do so at 8bpc.

Note that for simplicity we'll use the I2C access method for all dual
mode adaptors including type 2. Otherwise we'd have to start mixing
DP AUX and HDMI together. In the future we may need to do that if we
come across any board designs that don't hook up the DDC pins to the
DP++ connectors. Such boards would obviously only work with type 2
dual mode adaptors, and not type 1.

v2: Store adaptor type under indel_hdmi->dp_dual_mode
    Deal with DRM_DP_DUAL_MODE_UNKNOWN
    Pass adaptor type to drm_dp_dual_mode_max_tmds_clock(),
    and use it for type1 adaptors as well

Cc: stable@vger.kernel.org
Reported-by: Tore Anderson <tore@fud.no>
Fixes: 7a0baa62 ("Revert "drm/i915: Disable 12bpc hdmi for now"")
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462216105-20881-3-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Shashank Sharma <shashank.sharma@intel.com>