drm/exynos: fix opencoded use of drm_panel_*

drm/exynos: fix opencoded use of drm_panel_*

drm_panel_attach() will check if there is a controller
already attached - drop the check in the driver.

Use drm_panel_get_modes() so the driver no longer uses the function
pointer.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Kukjin Kim <kgene@kernel.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-3-sam@ravnborg.org