drm/i915: Make the high dword offset more explicit in i915_reg_read_ioctl

drm/i915: Make the high dword offset more explicit in i915_reg_read_ioctl

Store the upper dword of the register offset in the whitelist as well.
This would allow it to read register where the two halves aren't sitting
right next to each other, and it'll make it easier to make register
access type safe.

While at it change the register offsets to u32 from u64. Our register
space isn't quite that big, yet :)

v2: Use ldw/udw as the suffixes, and add a note about
    64bit wide split regs (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446839021-18599-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>