drm/i915: use macros to assign mmio access functions

This is beautification prep work since vgt will add even more special
cases. With these macros it's much easier to see what's going on
really.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
[danvet: #undef the temporary macros after the function again. And
write a commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>