video/rockchip: fix zero memory copy in vcodec reg_init

video/rockchip: fix zero memory copy in vcodec reg_init

When extra_size is zero, this triggers exception in kernel:

[ 2689.066416] rk_vcodec: set reg[77] ffffffff
[ 2689.066428] rk_vcodec: reg_copy_to_hw:1656: leave
[ 2689.066443] rk_iommu ff660480.iommu: Page fault at 0x0000000000000200 of type read
[ 2689.066452] rk_iommu ff660480.iommu: iova = 0x0000000000000200: dte_index: 0x0 pte_index: 0x0 page_offset: 0x200
[ 2689.066471] rk_iommu ff660480.iommu: mmu_dte_addr: 0x00000000785c4000 dte@0x00000000785c4000: 0x000000 valid: 0 pte@0x0000000000000000: 0x000000 valid: 0 page@0x0000000000000000 flags: 0x0
[ 2689.069518] rk_vcodec: try_set_reg:1738: leave
[ 2689.108387] rk_vcodec: vdpu_irq reg 1 status 408362 mask: irq 100 ready 1000 error 3e000
[ 2689.108406] rk_vcodec: vdpu_irq dec status 00408362
[ 2689.108413] rk_vcodec: task rkvdec status 00408362 mask 0003e000
[ 2689.108419] rk_vcodec: rkvdec task: 41 ms

This commit fixes that.