drm/exynos: fix no hdmi output

The hdmi outputs black screen only even though under the hood Xorg and
framebuffer console  are fine : devices found and initialized, but
not a pixel out.

Commit 93bca243 ("drm/exynos: remove struct exynos_drm_manager")
changed the call order of mixer_initialize with regards to
 exynos_drm_crtc_create.

This changes breaks hdmi out on Odroid U2 (linux-next with added
 Marek Szyprowski v4 hdmi patchset from linux-samsung-soc ML).

Restore the previous call ordering get hdmi to ouput proper pixels:
ie call mixer_initialize first then exynos_drm_crtc_create.

Fixes: 93bca243 ("drm/exynos: remove struct exynos_drm_manager")
Signed-off-by: Alban Browaeys <prahal@yahoo.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>