drm/msm: Remove the 'len' parameter from mmu map/unmap functions

Nobody uses this now that we are entirely scatter/gather enabled.
Remove it to avoid having to calculate the length unnecessarily.

Change-Id: Ic0dedbad020998e1c8fd5d526789f73beb0d5755
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>