cogl-winsys-egl-kms: Never set EGL_PLATFORM

This environment variable predates the reliable platform detection in mesa
and typically just causes crashes when the specified platform doesn't
match what's passed in.  Aside from being unecessary and problematic
it also leaks into the GNOME session, preventing clients from
automatically detecting the wayland platform.

https://bugzilla.gnome.org/show_bug.cgi?id=728978

Reviewed-by: Neil Roberts <neil@linux.intel.com>
cogl-winsys-egl-kms: Call swap notify handler even when pageflip ioctl fails

This can happen when we dpms off the output or when login1 takes away
drm master status from our drm fd.  In either case, we need to call
the swap notify handler so that the compositor dosn't get stuck waiting
for that notification.  The compositor should stop repainting shortly in
both cases, as it's either going into dpms off mode or vt switching away.

https://bugzilla.gnome.org/show_bug.cgi?id=728979

Reviewed-by: Neil Roberts <neil@linux.intel.com>