drm/i915: remove ibx_irq_uninstall

After the latest changes, ibx_irq_preinstall and ibx_irq_uninstall are
the same, so remove one of the copies and rename the other to
ibx_irq_reset (since we're using the "reset" name for things which are
called both at preinstall and uninstall).

v2: - Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>