drm/i915/dsc: Move rc param calculation for native_420

Move rc_param calculation for native_420 into calculate_rc_parameter.
second_line_bpg_offset and second_line_offset_adj are both rc params
and it would be better to have these calculated where all the other
rc parameters are calculated.

--v2
-Add the reason for commit in commit message [Jani]

--v3
-Move nsl_second_line_bpg_offset with the other 420 calculation
in calculate_rc_param [Ankit]

--v4
-Fix comment alignment [Ankit]

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230705051502.2568245-2-suraj.kandpal@intel.com