media: coda: bump maximum number of internal framebuffers to 19

While the h.264 standard only allows up to 16 reference frames, the CODA
firmware needs two more buffers: one to hold the currently decoded frame
and one for the display frame. Adding the framebuffer needed by the
driver for VDOA operation brings the total to a maximum of 19 internal
framebuffers.
Lift the current maximum of 17 internal framebuffers to allow playback
of high profile streams that require more than 14 reference frames.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>