drm/i915: use vfuncs for reg_read/write_fw_domains

Instead of going through the if-else chain every time, let's save the
function in the uncore structure. Note that the new functions are
purposely not used from the reg read/write functions to keep the
inlining there.

While at it, use the new macro to call the old ones to clean the code a
bit.

v2: Rename macros for no-forcewake function assignment (Tvrtko)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620010021.20637-2-daniele.ceraolospurio@intel.com