drm/msm/sde: keep minimum bus vote during client enable

Keep minimum bus vote during client enable instead
of 20M during parsing. This minimum bw votes keeps
refcount on MNOC GDSC and allows to turn on MDP
GDSC after it.

Change-Id: I0d09e1815b30bfa3d7080bb8c2bfc103826a2c48
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>