drm/msm: Implement msm drm fb_mmap callback function

This change implements msm drm specific fb_mmap function for fb device
to properly map the fb address to userspace.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com> (+ minor comment tweak)