drm/msm: Provide msm_gem_get_pages_locked()

Rename msm_gem_pin_pages_locked() to msm_gem_get_pages_locked(). The
function doesn't pin any pages, but only acquires them. Renaming the
function makes the old name available.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-4-tzimmermann@suse.de