panfrost: fix off-by-one when exporting format modifiers

panfrost: fix off-by-one when exporting format modifiers

`count` should not be incremented before the check, because it causes
the modifiers array to be filled starting from position 1 instead of 0.

This bug causes one less format modifier to be available than would
otherwise be expected, which could then lead to a dmabuf query failing
in situations where a supported modifier wouldn't be advertised.

It also causes garbage data to be advertised as a modifier in position 0
of the array, although this is not very likely to cause issues.

Fixes: 2a1217513 ("panfrost: Implement panfrost_query_dmabuf_modifiers")
Cc: mesa-stable
Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20879>
(cherry picked from commit 6c446377ff22d981191929de21cedecf519fe70f)