i915/drm: Remove two redundant agp_chipset_flushes

agp_chipset_flush() is for flushing the intel GMCH write cache via the
IFP, these two uses are for when we're getting the object into the cpu
READ domain, and thus should not be needed. This confused me when I was
getting my head around the code.

With thanks to airlied for helping me check my mental picture of how the
flushes and clflushes are supposed to be used.

Signed-off-by: Owain G. Ainsworth <oga@openbsd.org>
Signed-off-by: Eric Anholt <eric@anholt.net>