drm/exynos: use correct 'exynos-drm' name for platform device

Currently Exynos DRM driver uses DRIVER_NAME ('exynos') name for the
core platform device. This is confusing, because it doesn't refer to the
function the platform device is performing. This patch renames the
platform device to the 'exynos-drm', which matches the convention for
naming the platform devices. The name used inside DRM subsystem has not
been changed.

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