drm/amd/display: dc_assert_fp_enabled assert only if FPU is not enabled

Assert only when FPU is not enabled.

Fixes: 0ea7ee82

 ("drm/amd/display: Add DC_FP helper to check FPU state")
Signed-off-by: Anson Jacob <Anson.Jacob@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>