msm: mdss: Add check to fix null pointer dereference during pipe reset

During pipe reset, the mixer can be accessed without checking whether
it is allocated or not which can result in null pointer dereference.
This is fixed by adding a check before accessing the mixer.

CRs-Fixed: 1105018
Change-Id: If02d26102fedd9b7dc2a866cc7c6bcdaa447d4f7
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>