msm: mdss: Fix potential null pointer dereference

When resetting the mdp control there is a NULL check for left
mixer before calling pipe reset but the NULL check for right
mixer is missing causes a NULL pointer deferencing. This
change adds the NULL check.

Change-Id: Iebbc68c4016d8b13805187343eb7b657e8b76bcc
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>