msm: kgsl: Don't break up a secure buffer with 1MB mappings

For hardware reasons we can only map secure buffers with 1MB pages.
This contradicts 018a9fc8 but it
is preferred to have a possible GPU page fault over a kernel panic.

Change-Id: Ic0dedbad4cfcb7843e75aa6a0d9820b3a3bd07ae
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>