msm: mdss: Fix potential NULL pointer dereference

If ctl reset is called from rotator, ctl's mfd is NULL.
This might cause NULL pointer dereference leading to
kernel panic. Fix this by adding required check.

Change-Id: Icbd3e2e3ffc7cc0521222185d30ae0ca6adad5c4
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>