drm/i915/dp: Fix link compute m_n calc for DSC

Fix the intel_link_compute_m_n in case of display stream
compression. This patch passes the compressed_bpp to
intel_link_compute_m_n if compression is enabled.

Fixes: a4a15777 ("drm/i915/dp: Compute DSC pipe config in atomic check")
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181201010412.32372-1-manasi.d.navare@intel.com