DRM: Add DRM KMS/FB CMA helper

This patchset introduces a set of helper function for implementing the KMS
framebuffer layer for drivers which use the DRM GEM CMA helper function.

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