msm: mdss: hdmi: default pinctrl state to null

Default the pinctrl state to NULL before selecting
a pinctrl state. This ensures that IS_ERR_OR_NULL
called on the variable will return true.

Change-Id: I3e798cade45f4ef4179b1883e2eb33c3fd7d851f
Signed-off-by: Casey Piper <cpiper@codeaurora.org>