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>

drivers/gpu/drm/Kconfig

@@ -54,6 +54,12 @@ config DRM_TTM
 	  GPU memory types. Will be enabled automatically if a device driver
 	  uses it.
 
+config DRM_GEM_CMA_HELPER
+	bool
+	depends on DRM
+	help
+	  Choose this if you need the GEM CMA helper functions
+
 config DRM_TDFX
 	tristate "3dfx Banshee/Voodoo3+"
 	depends on DRM && PCI