drm/irq: Check for valid VBLANK before dereference

When accessing the array of per-CRTC VBLANK structures we must always
check that the index into the array is valid before dereferencing to
avoid crashing.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[danvet: Squash in my own whitespace ocd fixup in drm_vblank_count.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>