drm/i915: Fix PIPEDMC disabling for a bigjoiner configuration

For a bigjoiner configuration display->crtc_disable() will be called
first for the slave CRTCs and then for the master CRTC. However slave
CRTCs will be actually disabled only after the master CRTC is disabled
(from the encoder disable hooks called with the master CRTC state).
Hence the slave PIPEDMCs can be disabled only after the master CRTC is
disabled, make this so.

intel_encoders_post_pll_disable() must be called only for the master
CRTC, as for the other two encoder disable hooks. While at it fix this
up as well. This didn't cause a problem, since
intel_encoders_post_pll_disable() will call the corresponding hook only
for an encoder/connector connected to the given CRTC, however slave
CRTCs will have no associated encoder/connector.

Fixes: 3af2ff08 ("drm/i915: Enable a PIPEDMC whenever its corresponding pipe is enabled")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-2-imre.deak@intel.com