Revert "drm/msm: mark PM functions as __maybe_unused"

This reverts commit 78918cd0.

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: I14ac980cf68048111a7eebe49d124aa6a4dd0792
Signed-off-by: Samantha Tran <samtran@codeaurora.org>