msm: camera: isp: Use proportional UB slicing and 7 WM

The proportional slicing is used for determining Unified Buffer sizes
for each write master. This allows more efficiend use of UBs and
avoiding overflows.
Also this removes the need to set maximum number of write masters to
less than all available.

Change-Id: I774220268c96275b84d5fe265d68669b9fda4294
Signed-off-by: Petar Sivenov <psiven@codeaurora.org>