msm8974: mdss: dsi: uncached alloc for dsi buffer

Use uncached allocation for DSI buffers to avoid
cache inconsistency issues introduced with SLUB
DEBUG option enabled. With SLUB_DEBUG, the entries
in cache are somehow polluted which results in incorrect
dsi command values sent to the panel. This change avoids
cached allocation by using dma allocation instead.

Change-Id: Ib769e91d411bf856e7d2e860cff7c1188ff0236e
CRs-Fixed: 488236
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>