compositor-drm: correctly set the version of the drmEventContext

We implement v2 so use that instead of the DRM_EVENT_CONTEXT_VERSION
macro.

The latter defines the version of the drmEventContext struct declared in
the header [used in the current build] and can be 2, 3 or even 1000.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>