freedreno: reorder format check

With the addition of the planar formats helper, the
planar formats no longer have a valid block.bits field.
Calling util_format_get_blocksize therefore asserts.

Reorder the check to see if the format is supported
before doing the query to get the blocksize.

Fixes: 20f132e5 ("gallium/util: add planar format layouts and helpers")

Signed-off-by: Fritz Koenig <frkoenig@google.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
(cherry picked from commit c496d442)