common: edid: extract code for detailed timing search

Code which searches for valid detailed timing entry will be used in more
places. Extract it.

No functional change is made. However, descriptors are casted to
edid_detailed_timing instead of edid_monitor_descriptor. Descriptor can
be of either type, but since we're interested only in DTD, it is more
fitting to cast to edid_detailed_timing.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>