glamoregl: Initialize glamor on the main device

This allows e.g. an UDL device (driven by llvmpipe) to be automatically
set up with GPU acceleration via reverse PRIME.

The result is e.g.:

  # DISPLAY=:0.2 xrandr --listproviders
  Providers: number : 2
  Provider 0: id: 0xec cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 1 outputs: 1 associated providers: 1 name:modesetting
  Provider 1: id: 0x12c cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 2 associated providers: 1 name:Intel

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
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