drm/v3d: convert to use module_platform_driver

Get rid of boilerplate code by using module_platform_driver macro
for v3d_drm.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200810125931.186456-1-miaoqinglang@huawei.com