i965: Use 32 bit u_bit_scan for vertex attribute setup.

i965: Use 32 bit u_bit_scan for vertex attribute setup.

The vertex array object contains 32 vertex arrays. By that we cannot
reference more then these in the vertex shader inputs. So, we can use
the 32 bits u_bit_scan function to iterate the vertex shader inputs
and place an assert that only these are present. Also place an other
assert that the vertex array setup in i965 does not overrun the
enabled array in brw_context::vs::enabled.

v2: Style fixes.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/308>