drm/msm/dsi-staging: allocate memory for cmd dma in esd path

With continuous splash enabled, esd may send the first command
in kernel. Allocate the cmd tx memory buffer for esd path
if it is not yet allocated.

Change-Id: Ic94f9803413259b030e7512ed57028a628b8ef27
Signed-off-by: Vara Reddy <varar@codeaurora.org>