libdrm/intel: Make get_pipe_from_crtc_id per-bufmgr. Return -1 on failure.

The convention is that all APIs are per-bufmgr, so make this one the same.
Then, have it return -1 on failure so that the application can know what's
going on and do something sensible.

Signed-off-by: Keith Packard <keithp@keithp.com>