drm/omap: partial workaround for DRA7xx DMM errata i878

Errata i878 says that MPU should not be used to access RAM and DMM at
the same time. As it's not possible to prevent MPU accessing RAM, we
need to access DMM via a proxy.

This patch changes DMM driver to access DMM registers via sDMA. Instead
of doing a normal readl/writel call to read/write a register, we use
sDMA to copy 4 bytes from/to the DMM registers.

This patch provides only a partial workaround for i878, as not only DMM
register reads/writes are affected, but also accesses to the DMM mapped
buffers (framebuffers, usually).

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>