drm/exynos: Remove dummy encoder get_crtc operation implementation

drm/exynos: Remove dummy encoder get_crtc operation implementation

The encoder get_crtc operation is called to retrieve a pointer to the
CRTC the encoder is currenctly connected to, right after setting the
encoder::crtc field to the new CRTC. The implementation of this
operation returns the pointer to the new CRTC, which is then pointlessly
compared to itself.

As the operation is not mandatory, don't implement it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>