drm/rockchip: avoid duplicate mappings for IOMMU devices

If a buffer is allocated with alloc_kmap, then it is vmap'd on creation
and there is no reason to map it again in rockchip_gem_prime_vmap() when
the existing mapping can be used.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221110172415.2853420-1-john@metanate.com