i965: Bump GL_MAX_CUBE_MAP_TEXTURE_SIZE to 8192.

i965: Bump GL_MAX_CUBE_MAP_TEXTURE_SIZE to 8192.

Gen4+ supports 8192x8192 cube maps.  Ivybridge and later can actually
support 16384, but that would place GL_MAX_CUBE_MAP_TEXTURE_SIZE above
GL_MAX_TEXTURE_SIZE, which seems like a bad idea.

(Unfortunately, we can't bump GL_MAX_TEXTURE_SIZE to 16384 without
causing regressions due to awful W-tiled stencil buffer interactions.)

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>