i965: Don't use message headers for untyped reads

i965: Don't use message headers for untyped reads

We always set the mask to 0xffff, which is what it defaults to when no
header is present. Let's drop the header instead.

v2: Only remove header for untyped reads. Typed reads always need the
    header.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>