VSPD and VSP-DL devices can provide extended display lists supporting
extended command display list objects.

These extended commands require their own dma memory areas for a header
and body specific to the command type.

Implement a command pool to allocate all necessary memory in a single
DMA allocation to reduce pressure on the TLB, and provide convenient
re-usable command objects for the entities to utilise.
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>