drm/i915: Clean up M/N register defines

Use REG_GENMASK() & co. for the M/N register values. There are
also a lot of weird unused defines (eg. *_OFFSET) we can just
throw out.

Also let's mask out the unused bits during readout for good
measure. Previously we only masked out the TU_SIZE from one
of the registers, which was a bit too inconsistent for my
taste.

v2: Mention the readout masking in the commit msg  (Jani)
    Deal wth gvt

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220127120219.20143-1-ville.syrjala@linux.intel.com