drm/msm/disp/dpu: Remove unused code from drm_encoder.c

Remove dpu_encoder_check_mode and dpu_encoder_helper_hw_release
frmo drm_encoder.c as they appear to be unused.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>