gbm: Add map/unmap functions

gbm: Add map/unmap functions

This adds map and unmap functions to GBM utilizing the DRIimage extension
mapImage/unmapImage functions or existing internal mapping for dumb
buffers. Unlike prior attempts, this version provides a region to map and
usage flags for the mapping. The operation follows the same semantics as
the gallium transfer_map() function.

This was tested with GBM based gralloc on Android.

Signed-off-by: Rob Herring <robh@kernel.org>
[Emil Velikov: drop no longer relevant hunk from commit message.]
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>