drm/i915/dsc: move slice height calculation to encoder

Turns out this isn't compatible with DSI, where we use the value from
VBT. No functional changes.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3dd689688a51daff26088eaf6feac27f8b9f5ebc.1575974743.git.jani.nikula@intel.com