V4L2: Increase the number of image buffers for high fps

Increase the number of image buffers available to improve
performance at high framerates.

Signed-off-by: Dave Stevenson <dsteve@broadcom.com>
V4L2: Increase threshold for switching between video & stills

The threshold used to be hard and fast at 1280x720 and failed if
either width or height exceeded those limits. Now it limits it at
a pixel count of 1280*720 (ie 921600), so 1024x768 is now considered
a video mode.

Signed-off-by: Dave Stevenson <dsteve@broadcom.com>