drm/i915: Extract common MMIO lines

Just to make the churn and code duplication in upcoming patches a bit
less, turn code which is common to all GEN MMIO functions into a macro.

v2: Fix typo in subject

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>