drm/msm/sde: fix race condition causing undervote for bw and clk

Current driver checks for the bw and clock values for
the incoming frame in the case of a second interface,
this is wrong, as driver should be checking for current
voted value. Also, when checking these values between
multiple display in concurrent scenarios, driver needs
to protect the update with a mutex.

Change-Id: If26eb819b13a01b0be62cf198dc1c3e101f18720
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>