drm/i915: Nuke lspcon_downsampling

crtc_state->lspcon_downsampling isn't particularly useful at
the moment since we can't even do proper readout for it.
Let's get rid of it. Will help with unifying the LSPCON with
the regular DFP YCbCr output support.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924184156.24491-1-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>