drm/exynos: fix display power call issue.

Changelog v3:
make exynos_drm_display_power function use exynos_drm_best_encoder
function instead of connector->encoder to get a valid encoder.
connector->encoder could be NULL because with DRM_IOCTL_MODE_RMFB
request, connector->encoder is NULL so fix this issue.

Changelog v2:
remove static prefix so that exynos_drm_best_encoder function
could be called by other modules.

Changelog v1:
this patch fixes the issue that display power callback isn't called.
with DRM_IOCTL_MODE_RMFB request, encoder->connector becomes NULL
so display_ops->power_on() wouldn't be called so this patch makes
exynos_drm_best_encoder function to be used to get a valid encoder
of each connector.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>