PRIME: Don't advertise offload capabilities when acceleration is disabled

Xorg tends to crash if the user tries to actually use the offload
capabilities with acceleration disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57200
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
radeon/vdpau: don't report VDPAU for < r300

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>
glamor: enable DRI3

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>
radeon/dri3: support DRI3 with EXA acceleration

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>
Signed-off-by: Axel Davy <axel.davy@ens.fr>