drm/amd/display: Reject too small viewport size when validating plane

[why]
Overlay won't move to a new positon if viewport size is smaller than
what can be handled. It'd either disappear or stay at the old
position. This condition is for example hit if overlay is moved too
much outside of left or top edge of the screen, but it applies to
any non-cursor plane type.

[how]
Reject this contidion at validation time. This gives the calling
level a chance to handle this gracefully and avoid inconsistent
behaivor.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>