drm/edid: Add alternate clock for SMPTE 4K

[Why]
In hdmi_mode_alternate_clock(), it adds an exception for VIC 4
mode (4096x2160@24) due to there is no alternate clock defined for
that mode in HDMI1.4b. But HDMI2.0 adds 23.98Hz for that mode.

[How]
Remove the exception

v2: Adjust the comment description of hdmi_mode_alternate_clock()
due to there is no more exception for VIC 4 mode.

Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118101832.15487-2-Wayne.Lin@amd.com