drm/msm/sde: support to control layer mixer usage

This change enables resource manager to select the number of
layer mixers to be reserved for a particular usecase. Number of mixers
used will be controlled by the request from user by setting a
SDE_RM_TOPCTL_FORCE_MIXER_CFG property.

if SDE_RM_TOPCTL_FORCE_MIXER_CFG is set then
         num_lm = 1

Change-Id: I94d37cfd1632e4b8d42235c2c155d9cf6cf25fc2
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>