drm/tegra: Store pointer to vmap'ed framebuffer in screen_buffer

Tegra uses DMA-able memory, which has to be acessed with CPU ops
for system-memory. Store the framebuffer's vmap address in struct
fb_info.screen_buffer. The currently used field 'screen_base' is
for I/O memory.

Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-6-tzimmermann@suse.de