drm/amd/display: Trivial swizzle-related code clean-ups

drm/amd/display: Trivial swizzle-related code clean-ups

This is a very trivial code clean-up related to commit 5468c36d6285
("drm/amd/display: Filter Invalid 420 Modes for HDMI TMDS"). This commit
added a validation on driver probe to prevent invalid TMDS modes, but one
of the fake properties (swizzle) ended-up causing a warning on driver
probe; was reported here: https://gitlab.freedesktop.org/drm/amd/-/issues/2264.

It was fixed by commit a1cbe6916f44 ("drm/amd/display: patch cases with
unknown plane state to prevent warning"), but the validation code had
a double variable assignment, which we hereby remove. Also, the fix relies
in the dcn2{0,1}patch_unknown_plane_state() callbacks, so while at it we
took the opportunity to perform a small code clean-up in such routines.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Daniel Wheeler <daniel.wheeler@amd.com>
Cc: Fangzhi Zuo <Jerry.Zuo@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Mark Broadworth <mark.broadworth@amd.com>
Cc: Melissa Wen <mwen@igalia.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Sung Joon Kim <Sungjoon.Kim@amd.com>
Cc: Swapnil Patel <Swapnil.Patel@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>