[media] omap3isp: preview: Rename min/max input/output sizes defines

The macros that define the minimum/maximum input and output sizes are
defined in seperate files and have no consistent naming. In preparation
for preview engine cropping support, move them all to isppreview.c and
rename them to PREV_{MIN|MAX}_{IN|OUT}_{WIDTH|HEIGHT}*.

Remove unused and/or unneeded local variables that store the maximum
output width.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>