msm: mdss: Validate mixer/ctl are not null

mdss_mdp_wb_mixer_destroy could theoretically be
called with null mixer or ctl, which would cause
a null pointer dereference. Adding a check to
prevent this from happening.

Change-Id: I376c449daf05c8ca4ebb789d402575598ee59d86
CRs-Fixed: 551787
Signed-off-by: Casey Piper <cpiper@codeaurora.org>