helpers: Use backend num_planes_from_modifier only when modifier set

helpers: Use backend num_planes_from_modifier only when modifier set

If modifiers are not set, or if only linear modifier is set, just return
the number of planes based on the format. If there are other modifiers
set, then query the backend.

BUG=b:172830088
TEST=Start camera on zork after mesa uprev

Change-Id: I130e9ef3730b35f6e8777ce9c1919de2eb466733
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/2528204
Reviewed-by: Bas Nieuwenhuizen <basni@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Tested-by: Drew Davenport <ddavenport@chromium.org>
Commit-Queue: Drew Davenport <ddavenport@chromium.org>