drm/exynos: return return value of exynos_crtc->enable_vblank
drm/exynos: return return value of exynos_crtc->enable_vblank

Instead of blindly ignore the return value of enable_vblank return it
to the upper DRM layer for error handling.

Suggested-by: Joonyoung Shim <b2207686f8c5343cf0f505ae1d5720ecb0c3202a@samsung.com>
Signed-off-by: Gustavo Padovan <029a69b25713d0799b81c0a39685b4c040fa1425@collabora.co.uk>
Reviewed-by: Joonyoung Shim <b2207686f8c5343cf0f505ae1d5720ecb0c3202a@samsung.com>
Signed-off-by: Inki Dae <bae4a7a27bbc965064e97edd6e179ac4979db440@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs