drm/i915: Fix enabled infoframe states of lspcon

Compared to implementation of DP and HDMI's encoder->infoframes_enabled,
the lspcon's implementation returns its active state. (we expect enabled
infoframe states of HW.) It leads to pipe state mismatch error
when ddi_get_config is called.

Because the current implementation of lspcon is not ready to support
readout infoframes, we need to return 0 here.

In order to support readout to lspcon, we need to implement read_infoframe
and infoframes_enabled. And set_infoframes also have to set an appropriate
bit on crtc_state->infoframes.enable

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514060732.3378396-11-gwan-gyeong.mun@intel.com