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

This reverts commit 2c7b48e7.

This reversion is happening as a result of disabling the
disp/dpu directory as it will not be used in 4.19 downstream
display driver and these changes are made in the common layer
which affects the downstream driver functionalities. Separate
patches are submitted to revert the upstream driver and
introduce the downstream version of display driver.

Change-Id: Ic432588c26ac1027a8886662be931119d5f4468e
Signed-off-by: Samantha Tran <samtran@codeaurora.org>