drm/amd/display: Remove semicolon from to_dm_plane_state definition

The extra ; in the macro definition creates an empty statement
preventing any variable declarations from occuring after
any use of to_dm_plane_state(...).

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>