msm: camera: Increase maximum number of writemasters

Video Front End (VFE) HW v4.0 can support up to 7 writemasters.
Currently we support only 4 in order to preserve latency
buffers for each writemasters. This change increase the
maxmimum number of writemasters supported to support
simultaneous camera in single VFE.

Change-Id: I168d1a4df23ed65ae8f8638c30cde7b2a4f044c5
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>