drm/gem-dma: set VM_DONTDUMP for mmap

drm/gem-dma: set VM_DONTDUMP for mmap

When the mmap function was converted from a file op to a GEM object
function in commit f5ca8eb6f9bd ("drm/cma-helper: Implement mmap as GEM
CMA object functions") some VM flags were not lifted from drm_gem_mmap():

  - VM_IO
  - VM_DONTEXPAND
  - VM_DONTDUMP

VM_DONTEXPAND was added back in commit 59f39bfa6553 ("drm/cma-helper:
Set VM_DONTEXPAND for mmap"). VM_IO doesn't make sense since these are
memory buffers, while "IO tells people not to look at these pages
(accesses can have side effects)".

Add back VM_DONTDUMP. This matches the behavior of most other GEM
implementations.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260317040034.617585-1-wenst@chromium.org
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>