drm/amd/display: Reduce frame size in the bouding box for DCN21

drm/amd/display: Reduce frame size in the bouding box for DCN21

GCC throw warnings for the function dcn21_update_bw_bounding_box and
dcn316_update_bw_bounding_box due to its frame size that looks like
this:

 error: the frame size of 1936 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

For fixing this issue I dropped an intermadiate variable.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>