drm/amd/display: Fix unintialized max_bpc state values

[Why]
If the "max bpc" isn't explicitly set in the atomic state then it
have a value of 0. This has the correct behavior of limiting a panel
to 8bpc in the case where the panel supports 8bpc. In the case of eDP
panels this isn't a true assumption - there are panels that can only
do 6bpc.

Banding occurs for these displays.

[How]
Initialize the max_bpc when the connector resets to 8bpc. Also carry
over the value when the state is duplicated.

Bugzilla: https://bugs.freedesktop.org/108825
Fixes: 307638884f72 ("drm/amd/display: Support amdgpu "max bpc" connector property")
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -3218,6 +3218,7 @@ void amdgpu_dm_connector_funcs_reset(struct drm_connector *connector)
 		state->underscan_enable = false;
 		state->underscan_hborder = 0;
 		state->underscan_vborder = 0;
+		state->max_bpc = 8;
 
 		__drm_atomic_helper_connector_reset(connector, &state->base);
 	}
@@ -3239,6 +3240,7 @@ amdgpu_dm_connector_atomic_duplicate_state(struct drm_connector *connector)
 
 	new_state->freesync_capable = state->freesync_capable;
 	new_state->abm_level = state->abm_level;
+	new_state->max_bpc = state->max_bpc;
 
 	return &new_state->base;
 }