media: imx: Set default sizes through macros in all drivers

All drivers use 640x480 as the default size, but they all hardcode those
values. Create two global macros named IMX_MEDIA_DEF_PIX_WIDTH and
IMX_MEDIA_DEF_PIX_HEIGHT to store the default size, and use them through
the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>