vc_mem: Add the DMA memcpy support from bcm2708_fb

bcm2708_fb is disabled by the vc4-kms-v3d overlay, which means that the
DMA memcpy support it provides is not available to allow vclog to read
the VC logs from the top 16MB on Pi 2 and Pi 3. Add the code to the
vc_mem driver, which will still be enabled.

It ought to be possible to do a proper DMA_MEM_TO_MEM copy via the
generic DMA customer API, but that can be a later step.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>