DRM: Add DRM GEM CMA helper

Many embedded drm devices do not have a IOMMU and no dedicated
memory for graphics. These devices use CMA (Contiguous Memory
Allocator) backed graphics memory. This patch provides helper
functions to be able to share the code. The code technically does
not depend on CMA as the backend allocator, the name has been chosen
because CMA makes for a nice, short but still descriptive function
prefix.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
[Make DRM_GEM_CMA_HELPER a boolean Kconfig option]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>