drm: omapdrm: Support exporting of non-contiguous GEM BOs

drm: omapdrm: Support exporting of non-contiguous GEM BOs

Currently code allocates non-scanout BOs from SHMEM and those objects are
accessible to userspace by mmap(). However, on devices with no DMM (like
OMAP3), the same objects are not accessible by kernel drivers that want to
render to them as code refuses to export them. In turn this means that on
devices with no DMM, all buffers must be allocated as scanout, otherwise
only CPU can access them. On those devices, scanout buffers are allocated
from CMA, making those allocations highly unreliable.

Fix that by implementing functionality to export SHMEM backed buffers on
devices with no DMM. This makes CMA memory only being used when needed,
instead for every buffer that has to be off-CPU rendered.

Tested on Motorola Droid4 and Nokia N900

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1642587791-13222-3-git-send-email-ivo.g.dimitrov.75@gmail.com