drm: amd/display: fix spelling of "function"

Fix spellos of "function" in drivers/gpu/drm/amd/display/.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>