mesa: Fix the datatype of GL_DEPTH32F_STENCIL8's depth channel.

mesa: Fix the datatype of GL_DEPTH32F_STENCIL8's depth channel.

Asking for the datatype of MESA_FORMAT_Z32_FLOAT_X24S8 is a bit funny
-- there's a float depth channel, and a stencil channel that doesn't
have a particular GLenum associated with its type, so what's the
correct response?

Because there is no query for stencil, just make this format's
datatype be that of the depth channel.  It fixes the depth query (and
thus a failure in piglit gl-3.0-required-sized-formats), and none of
the other consumers of the _mesa_get_format_datatype() API care.

v2: Add a comment for why the DataType is this way for this format.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>