drm/amd/display: Add I2C memory low power support

[Why]
The I2C memory blocks should be powered down when they are not in use.
This will reduce power consumption.

[How]
1. Write to I2C_LIGHT_SLEEP_FORCE to put memory in light sleep when
   released
2. Added a debug option to allow this behaviour to be turned off

Signed-off-by: Jacky Liao <ziyu.liao@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>