drm/amd/display: fix static checker warning

This patch fixes static checker warning of
"warn: cast after binop" introduced by
56087b31 drm/amd/display: fix high part address in dm_plane_helper_prepare_fb()

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>