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>

src/mesa/drivers/dri/i965/brw_context.c

@@ -313,6 +313,7 @@ brw_initialize_context_constants(struct brw_context *brw)
       ctx->Const.MaxTextureLevels = MAX_TEXTURE_LEVELS;
    ctx->Const.Max3DTextureLevels = 12; /* 2048 */
    ctx->Const.MaxCubeTextureLevels = 14; /* 8192 */
+   ctx->Const.MaxTextureMbytes = 1536;
 
    if (brw->gen >= 7)
       ctx->Const.MaxArrayTextureLayers = 2048;