msm: mdss: edid: conditionally set rgb_support for Y420 EDID modes

As part of Y420VDB, an EDID mode might have been already added to the
mode_list supporting only Y420 sampling mode. And later while parsing VDB
block, for such EDID modes, set rgb_support flag only if its TMDS clock
is within max TMDS clock.

Change-Id: Ieaa9e0fc82d1fd81f8ea07829da6c1b32ef6018a
Signed-off-by: Narender Ankam <nankam@codeaurora.org>