drm/msm/sde: add connector support for power modes

This change enables a new LP property to specify panel specific
low power modes. This is needed to differentiate between normal
panel "on" and "on but low power" scenarios, something that
is not available through the standard DRM DPMS property.

The sde connector calculates a consolidated "power mode" setting
between the DPMS and LP properties and provides a callback to
the underlying display driver(s) whenever one of them is updated.

CRs-Fixed: 2027543
Change-Id: If5c80ac9eefbf1f119bcae5513ae18c7be6f618d
Signed-off-by: Clarence Ip <cip@codeaurora.org>