drm/amd/display: Use drm helper for resetting plane state

[Why]
To help prevent plane state not being set to the correct default
value if any new properties are added in the future.

[How]
Use the drm helper - which seems to be the common solution among other
DRM drivers.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>