msm: mdss: Prevent mixer GC programming if it is currently enabled

The layer mixer GC hardware is meant to be programmed and enabled only
once. In cases where we want to reprogram the values in the hardware,
there is the possibility of corruption. This change forces the mixer
setup to skip GC programming if it is currently enabled. The user
should disable the feature before enabling it with different values.

Change-Id: I3c049d87a230be6f65ffa354099a2f1f1096f78b
Signed-off-by: Benet Clark <benetc@codeaurora.org>