mdss: dsi: Resolving memory leak in parse dsi commands

Free buf variable before exiting function in error cases
in order to avoid possible memory leaks.

Change-Id: I8ebc3f0f3f40be85e312f9646374a074f1414bdb
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>