drm/irq: Make pipe unsigned and name consistent

Name all references to the pipe number (CRTC index) consistently to make
it easier to distinguish which is a pipe number and which is a pointer
to struct drm_crtc.

While at it also make all references to the pipe number unsigned because
there is no longer any reason why it should ever be negative.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>