drm/amd/display: Consolidate HPO enable/disable and restrict only to state transitions.

drm/amd/display: Consolidate HPO enable/disable and restrict only to state transitions.

[WHY]
Previously, we'd disabled HPO whenever an HPO display was disconnected. This
caused other HPO displays to blank whenever one was unplugged.

[HOW]
This change restricts HPO enable/disable to dce110_apply_ctx_to_hw and adds a
helper function (dce110_is_hpo_enabled) that returns true if any HPO displays
are present in a context. We compare the current and previous dc ctx to check
whether HPO is transitioning from on to off or vice versa, and adjust the HPO
state accordingly.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Natanel Roizenman <natanel.roizenman@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>