drm/exynos: fix struct for operation callback functions to driver name

drm/exynos: fix struct for operation callback functions to driver name

The mixer driver and hdmi driver have each operation callback functions
and they is registered to hdmi common driver. Their struct names in hdmi
common driver include display, manager and overlay. It confuses to
appear whose operation and two driver cannot register same operation
callback functions at the same time. Use their struct names to driver
name.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>