p1-common: libcamera: reduce fimc buffer

althought 720P isn't working yet but 8 is enough as well
for 720P video capture
this patch save memory by reducing from 9 to 8 fimc buffers

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I27e54d7b43168c66ac886ae32bfe6496829affaf