i965: Fix assertion in brw_reg_type_letters

i965: Fix assertion in brw_reg_type_letters

While using various debugging features (optimization debug, instruction dumping,
etc) this function is called in order to get a readable letter for the type of
unit.

On GEN8, two new units were added, the Qword and the Unsigned Qword (Q, and UQ
respectively). The existing assertion tries to determine that the argument
passed in is within the correct boundary, however, it was using UQ as the upper
limit instead of Q.

To my knowledge you can only hit this case with the branch I am currently
working on, so it doesn't fix any known issues.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>