drm/omap: Add support for render nodes

Add support for render nodes in omap driver and allow required
ioctls to be accessible via render nodes.

This enables unprivileged clients to allocate resources like GEM buffers
for rendering their content into. Mode setting (KMS ioctls) is not
allowed using render nodes. These buffers are then shared with
a previleged process (e.g compositor) that has mode setting access.

An example of this use case is Android where the hardware composer is
the only master and has mode setting access. Every other client then
uses render node(e.g /dev/dri/renderD128 to allocate and use its buffers.

Signed-off-by: Hemant Hariyani <hemanthariyani@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>