drm/mode: get the modeline for standard timing in EDID by using CVT/GTF

Create the standard timing modeline by using CVT/GFT algorithm while
interpreting the EDID.

In course of interpreting the EDID, the timing level will be obtained,
which is used to determine whether the CVT/GTF algorithm is selected to
generate the modeline for the given hdisplay/vdisplay/vrefresh_rate.
In the UMS mode firstly it will check whether it can be found in
the DMT table. If it can be found, then the modeline is returned. Then the
timing_level is used to choose CVT/GTF.
As there is no DMT table, no modeline is returned when timing level
is DMT. For the other two timing levels, the CVT/GTF will be called to
generate the required standard timing modeline.

[airlied: fixed up conflicts since EDID rework]

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>