drm/msm/dpu: Remove dead-code in dpu_encoder_helper_reset_mixers()

drm/msm/dpu: Remove dead-code in dpu_encoder_helper_reset_mixers()

'mixer' is only zeroed and is not use. Remove it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Acked-By: Mahesh Bharadwaj Kannan <mahesh.kannan@oss.qualcomm.com>
Fixes: ae4d721ce100 ("drm/msm/dpu: add an API to reset the encoder related hw blocks")
Patchwork: https://patchwork.freedesktop.org/patch/679854/
Link: https://lore.kernel.org/r/8e3b2fbbf5440aa219feb667f5423c7479eb2656.1760040536.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>