drm/msm/dpu: Remove dpu_crtc_get_mixer_height

dpu_crtc_get_mixer_height() is only used once and the value it
returns can be easily derived from the calling function.

v3: No changes

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>