egl/dri2: set max values for pbuffer width and height

egl/dri2: set max values for pbuffer width and height

While these max values were previously fixed for pbuffer creation, this
change makes also eglGetConfigAttrib() return correct values.

Fixes following dEQP tests:

   dEQP-EGL.functional.create_surface.pbuffer.rgb888_no_depth_no_stencil
   dEQP-EGL.functional.create_surface.pbuffer.rgb888_depth_stencil
   dEQP-EGL.functional.create_surface.pbuffer.rgba8888_no_depth_no_stencil
   dEQP-EGL.functional.create_surface.pbuffer.rgba8888_depth_stencil

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98326
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "12.0 13.0" <mesa-stable@lists.freedesktop.org>