msm: kgsl: drm: Increased max sub-buffers allowed to 3

To permit triple buffering, number of allowed sub-buffers
was increased to 3 from 2.

Signed-off-by: Michael Street <mstreet@codeaurora.org>
Change-Id: I0d70df7b1ecad438280c9560d9b70354f3681d8f