drm/amd/display: refactor dirty rect dmub command decision

[Why]
To wrap the decision logic of sending dirty rect dmub command
for both frame update and cursor update path.

Signed-off-by: Robin Chen <po-tchen@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>