drm/i915: Skip register reads in i915_get_crtc_scanoutpos()

We have all the information we need in the mode structure, so going and
reading it from the hardware is pointless, and slower.

We never populated ->get_vblank_timestamp() in the UMS case, and as that
is the only way we'd ever call ->get_scanout_position(), we can
completely ignore UMS in i915_get_crtc_scanoutpos().

Also reorganize intel_irq_init() a bit to clarify the KMS vs. UMS
situation.

v2: Drop UMS code

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by:  <mario.kleiner.de@gmail.com>
Tested-by:  <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>