drm/vram-helper: Reuse existing page mappings in vmap

For performance, BO page mappings can stay in place even if the
map counter has returned to 0. In these cases, the existing page
mapping has to be reused by the next vmap operation. Otherwise
a new mapping would be installed and the old mapping's pages leak.

Fix the issue by reusing existing page mappings for vmap operations.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 1086db71 ("drm/vram-helper: Remove invariant parameters from internal kmap function")
Acked-by: Christian König <christian.koenig@amd.com>
Tested-by: Eli Cohen <elic@nvidia.com>
Reported-by: Eli Cohen <elic@nvidia.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210118144639.27307-1-tzimmermann@suse.de