drm/virtio: Use generic dumb_map_offset implementation

Currently, virtio uses its own dumb_map_offset implementation,
virtio_gpu_mode_dumb_mmap. It works similarly to generic implementation,
drm_gem_dumb_map_offset, and using the generic implementation is
preferable (and making drivers to do so is a task stated on the DRM
subsystem's TODO list).

Thus, make driver use the generic implementation. This includes
VIRTGPU_MAP ioctl so it cannot be used to circumvent rules imposed by
drm_gem_dumb_map_offset (imported objects cannot be mapped).

Signed-off-by: Peter Shkenev <mustela@erminea.space>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
[dmitry.osipenko@collabora.com: cosmetic code improvements]
Link: https://patchwork.freedesktop.org/patch/msgid/20241107141133.13624-1-mustela@erminea.space