disp: msm: dp: fix mode filtering for 4K120

Currently, 4K120 mode is being filtered by DP driver
due to invalid topology assignation. When 4 LMs are
needed, the number of DSCs must match the number of
LMs since only DSCMERGE based topologies are supported
for driving DSC capable panels.

This change fixes the topology validation logic and
aggregates LM and DSC validation functions in the topology
one since the number of LMs and DSCs must now be considered
in calculating the topology requirement.

Change-Id: I95577eabb34068cd490d23e70ffbb899d1c2d03a
Signed-off-by: Amine Najahi <anajahi@codeaurora.org>