mesa: fix fallthrough in glformats

mesa: fix fallthrough in glformats

Before 908f817918fb this would fallthrough to GL_INVALID_OPERATION
if the validation condition was not met. But since that change it
will now only return GL_INVALID_OPERATION if
!_mesa_has_EXT_texture_compression_bptc(ctx) is true. This seems
unintended.

Here we fix up the fallthrough and add the fallthrough comment so
this doesn't happen again.

Fixes: 908f817918fb ("mesa: expose EXT_texture_compression_bptc in GLES")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3005
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5705>