drm/i915/display/dsc: Refactor intel_dp_dsc_compute_bpp

Move the platform specific max bpc calculation into
intel_dp_dsc_compute_bpp function

Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514153711.2359617-4-matthew.d.roper@intel.com