drm/i915/gtt: Introduce kmap|kunmap for dma page

As there is flushing involved when we have done the cpu
write, make functions for mapping for cpu space. Make macros
to map any type of paging structure.

v2: Make it clear tha flushing kunmap is only for ppgtt (Ville)
v3: Flushing fixed (Ville, Michel). Removed superfluous semicolon

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>