drm/msm: Add hint to DRM_IOCTL_MSM_GEM_INFO to return an object IOVA

Modify the 'pad' member of struct drm_msm_gem_info to 'flags'. If the
user sets 'flags' to non-zero it means that they want a IOVA for the
GEM object instead of a mmap() offset. Return the iova in the 'offset'
member.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
[robclark: s/hint/flags in commit msg]
Signed-off-by: Rob Clark <robdclark@gmail.com>