drm/rockchip: fix fbdev on non-IOMMU devices

When switching to the generic fbdev infrastructure, it was missed that
framebuffers were created with the alloc_kmap parameter to
rockchip_gem_create_object() set to true.  The generic infrastructure
calls this via the .dumb_create() driver operation and thus creates a
buffer without an associated kmap.

alloc_kmap only makes a difference on devices without an IOMMU, but when
it is missing rockchip_gem_prime_vmap() fails and the framebuffer cannot
be used.

Detect the case where a buffer is being allocated for the framebuffer
and ensure a kernel mapping is created in this case.

Fixes: 24af7c34 ("drm/rockchip: use generic fbdev setup")
Reported-by: Johan Jonker <jbx6244@gmail.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221020181248.2497065-1-john@metanate.com