drm/msm: Pass the correct aperture end to drm_mm_init

drm_mm_init() takes the start and length of the intended virtual
memory address region but the msm code is passing the end of
the region instead. That would work out if the region started
at 0 but it doesn't so the top of the region sneaks above the
32 bit boundary which won't work because the driver doesn't
support 64 bit addresses for the GPU yet.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>