drm/msm: set dma_mask properly

Previous value really only made sense on armv7 without LPAE.  Everything
that supports more than 4g of memory also has iommu's that can map
anything.

Change-Id: Ic0dedbad6ca5973f2ac0d2690dec77e40787b8da
Git-commit: 49d24b7a6bf253aeb7cb81672316c83e7b628ea9
Git-repo: https://github.com/freedreno/kernel-msm


Signed-off-by: Rob Clark <robdclark@gmail.com>
[jcrouse@codeaurora.org: Fixed minor merge issues]
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>