Use correct glGetGraphicsResetStatus()

KWin loads glGetGraphicsResetStatus() on its own to handle ARB and EXT
extensions. After moving graphics reset handling to the RenderBackend,
kwin doesn't use its own glGetGraphicsResetStatus() function, but instead
uses the one provided by libepoxy, which panics if
glGetGraphicsResetStatus() is not in core spec. This change makes the
OpenGLBackend use kwin's glGetGraphicsResetStatus() function instead of
the one provided by libepoxy to avoid a crash.