drm/gem-vram: Do not pin buffer objects for vmap

Pin and vmap are distinct operations. Do not perform a pin as part
of the vmap call. This used to be necessary to keep the fbdev buffer
in place while it is being updated. Fbdev emulation has meanwhile
been fixed to lock the buffer correctly. Same for vunmap.

For refactoring the code, remove the pin calls from the helper's
vmap implementation in drm_gem_vram_vmap() and inline the call to
drm_gem_vram_kmap_locked(). This gives a vmap helper that only
maps the buffer object's memory pages without pinning or locking.
Do a similar refactoring for vunmap.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-13-tzimmermann@suse.de