drm/msm/gpu: Allocate the correct size for the GPU memptrs

Allocate the correct buffer size for the GPU memptrs. The incorrect
size hasn't affected us thus far since the incorrect size was larger
than the intended size and we're still stuck on page sized
granularity anyway but technically correct is the best kind of
correct.

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