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>

(cherry picked from commit 8fb9cea4515161effdaee256dd033c36cfa21689)

Note while forward-porting: On master this patch only affects the
Wayland winsys. The KMS winsys never set EGL_PLATFORM on master
because that was introduced in commit dab054200cb2135da2303 which was
only applied to the 1.18 branch. That patch was only made to override
the environment variable if the Wayland winsys is tried before the KMS
winsys.
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>
(cherry picked from commit 22378d572b039e4fe6ceb56e56492470bb461d48)