drm/exynos: g2d: change platform driver name to 'exynos-drm-g2d'

drm/exynos: g2d: change platform driver name to 'exynos-drm-g2d'

The current name is 's5p-g2d', which is identical with the driver
name of the old V4L2 driver in media/platform.
This is probably due to the DRM driver being based on the V4L2
driver when it was initially created. Still the clashing of driver
names is confusing, so rename it to something in line with the
other DRM subdrivers.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>