V4L2: Add option to disable enum_framesizes.

Gstreamer's handling of a driver that advertises
V4L2_FRMSIZE_TYPE_STEPWISE to define the supported
resolutions is broken. See bug
https://bugzilla.gnome.org/show_bug.cgi?id=726521

Optional parameter of gst_v4l2src_is_broken added.
If non-zero, the driver claims not to support that
ioctl, and gstreamer should be happy again (it
guesses a set of defaults for itself).

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