Adds system power management support in KMS kernel driver.

Depends on:
https://patchwork.freedesktop.org/series/62377/

Changes since v1:
Since we have unified mclk/pclk/pipeline->aclk to one mclk, which will
be turned on/off when crtc atomic enable/disable, removed runtime power
management.
Removes run time get/put related flow.
Adds to disable the aclk when register access finished.

Changes since v2:
Rebases to the drm-misc-next branch.
Signed-off-by: Lowry Li (Arm Technology China) <lowry.li@arm.com>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190923015908.26627-1-lowry.li@arm.com