drm/msm/mdp5: Fix typo in encoder_enable path

The mdp5_cmd_encoder_disable is accidentally called in the encoder enable
path. We've not seen any problems since we haven't tested with command
mode panels in a while. Fix the copy-paste error.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>