intel: Add a getter for the intel_context ctx_id

Exposing the u32 context ID makes it possible to define new drm kernel
interfaces based on the same IDs that e.g. execbuf uses to identify a
gem context, that aren't themselves abstracted by libdrm but need to be
used by libdrm/drm_intel_context based clients such as (parts of) i-g-t
or Mesa.

For example this can be used to configure an i915-perf stream to collect
metrics for a specific context.

v2: s/drm_intel_gem_context_get_context_id/drm_intel_gem_context_get_id/

Signed-off-by: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>