drm/radeon: Fix encoder->possible_clones

Include the encoder itself in its possible_clones bitmask.
In the past nothing validated that drivers were populating
possible_clones correctly, but that changed in commit
74d2aacbe840 ("drm: Validate encoder->possible_clones").
Looks like radeon never got the memo and is still not
following the rules 100% correctly.

This results in some warnings during driver initialization:
Bogus possible_clones: [ENCODER:46:TV-46] possible_clones=0x4 (full encoder mask=0x7)
WARNING: CPU: 0 PID: 170 at drivers/gpu/drm/drm_mode_config.c:615 drm_mode_config_validate+0x113/0x39c
...

Cc: Alex Deucher <[e-mail]>
Cc: [e-mail]
Fixes: 74d2aacbe840 ("drm: Validate encoder->possible_clones")
Reported-by: Erhard Furtner <[e-mail]>
Closes: https://lore.kernel.org/dri-devel/20241009000321.418e4294@yea/
Tested-by: Erhard Furtner <[e-mail]>
Signed-off-by: Ville Syrjälä <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit 3b6e7d40649c0d75572039aff9d0911864c689db)
Cc: [e-mail]

drivers/gpu/drm/radeon/radeon_encoders.c

···
 	struct radeon_device *rdev = dev->dev_private;
 	struct radeon_encoder *radeon_encoder = to_radeon_encoder(encoder);
 	struct drm_encoder *clone_encoder;
-	uint32_t index_mask = 0;
+	uint32_t index_mask = drm_encoder_mask(encoder);
 	int count;
 
 	/* DIG routing gets problematic */