drm/msm/dpu: Remove _dpu_crtc_power_enable

All checks for _dpu_crtc_power_enable are not true, so the function
can never return an error code. This removes the need for the
function as pm_runtime functions can be used instead.

v3: Separated _dpu_crtc_power_enable into _dpu_crtc_power_enable and
_dpu_crtc_power_disable for clarity.

v4: Removed both _dpu_crtc_power_enable and _dpu_crtc_power_disable
and called pm_runtime_get_sync and pm_runtime_put_sync from all
call points

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