msm: mdss: fix under-runs during suspend resume

When device goes into suspend, MDP driver will put pipeline into border
color only without staging any pipes. Similar pipeline is setup while
resuming the device. When MDP pipeline is in border color only, minimum
MDP clock requirement is based on mixer width. Currently MDP clock
calculations are skipped if number of layers staged are 0. This set MDP
to minimum clock rate and device under-runs. Fix this by allowing MDP
clock rate calculations even when number of staged layers are 0.

Change-Id: I1c00e00b0b4ac76866a7863293bf29d2bc6a423d
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>