drm/gem-vram: Acquire reservation lock in GEM pin/unpin callbacks

Acquire the reservation lock directly in GEM pin callback. Same for
unpin. Prepares for further changes.

Dma-buf locking semantics require callers to hold the buffer's
reservation lock when invoking the pin and unpin callbacks. Prepare
gem-vram accordingly by pushing locking out of the implementation.
A follow-up patch will fix locking for all GEM code at once.

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/20240227113853.8464-3-tzimmermann@suse.de