drm/irq: Make drm_vblank_pre/post_modeset internal

Now that all drivers are switched over to drm_crtc_vblank_on/off we
can relegate pre/post_modeset to the purely drm_irq.c internal role of
supporting old ums userspace.

As usual switch to the drm_legacy_ prefix to make it clear this is
for old drivers only.

v2: Rebase on top of Thierry's s/int crtc/unsigned int pipe/ changes.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>