drm/amd/display: Use atomic types for ref_count

Current ref_count inc/dec is not guarded by locks which leads to
a raced condition where two threads try to access the variable
at the same time. In this case, both might act on the same cached
value and inc/dec from the same value, rather than inc/dec by 2.

Signed-off-by: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>