drm/msm: Update the MMU helper function APIs

Instead of using a bare unsigned type for the length value for map/unmap
functions pass in a size_t to more correctly match up with the underlying
APIs.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>