i965: Bump MaxTexMbytes from 1GB to 1.5GB.

i965: Bump MaxTexMbytes from 1GB to 1.5GB.

Even with the other limits raised, TestProxyTexImage would still reject
textures > 1GB in size.  This is an artificial limit; nothing prevents
us from having a larger texture.  I stayed shy of 2GB to avoid the
larger-than-aperture situation.

For 3D textures, this raises the effective limit:
 - RGBA8:   645 -> 738
 - RGBA16:  512 -> 586
 - RGBA32F: 406 -> 465

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74130
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>