drm/msm: Allow exported dma-bufs to be mapped

Currently msm does not implement gem_prime_mmap. Without this it is not
possible to draw onto a dma-buf from userspace (making its very hard to
implement the Android rendering model).

Fixing this is just a matter of adding a little boilerplate.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>