drm/msm/sde: fix resource allocation for CWB

In current SDE driver CWB resource allocation is based on WB
connector mode. This leads to improper resource allocation
when an external display using single LM is connected and
primary is with dual LM. Current SDE driver assumes WB is with
single LM below a mixer width threshold and allows improper
resource allocation for CWB. This change overrides the WB topology
and allocates required number of layer-mixer and pingpong blocks
based on primary display topology.

Change-Id: Ia04996aac6ddecaad22c46fd9a37e7bc8a2b8355
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>