drm/amd/display: Minor cleanup for DCN401 cursor related code

drm/amd/display: Minor cleanup for DCN401 cursor related code

Move pipe_ctx variables to start of the function and add a helpful comment

Co-authored-by: Sridevi Arvindekar <sarvinde@amd.com>
Reviewed-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Sridevi Arvindekar <sarvinde@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>