[Intel-gfx] Page flip fixes for Ironlake

[Zhenyu Wang]

These two patches try to fix page flip issue seen on Ironlake,
to handle flip done irq properly and fix pipe source image setting
in flip command. 

They're based on Jesse's "drm/i915: untangle page flip completion"
which I hope to be merged ASAP. 

thanks.

p.s I've also tried to enable the flip done event mechanism on my 965GM
but failed, it looks not that simple to mask FLIP_PENDING bit in IMR
first and hoping it'll be unmasked when flip is performed...