egl: support EGL_LARGEST_PBUFFER in eglCreatePbufferSurface(...)

egl: support EGL_LARGEST_PBUFFER in eglCreatePbufferSurface(...)

Patch provides a default for a set pbuffer surface size when
EGL_LARGEST_PBUFFER is used by the client. MIN2 macro is moved
to egldefines so that it can be shared.

Fixes following Piglit test:
   egl-create-largest-pbuffer-surface

From EGL 1.5 spec:
   "Use EGL_LARGEST_PBUFFER to get the largest available pbuffer
   when the allocation of the pbuffer would otherwise fail."

Currently there exists no API to query largest available pixmap size
using xlib or xcb so right now this seems most straightforward way to
ensure that we fulfill above API and also we don't attempt to allocate
'too big' pixmap which might succeed on server side but not work in
practice when driver starts to use it as a texture.

v2: add more explanation about the change (Emil)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>