drm: tweak drm_print_bits()

There is little reason for the from/to logic, printing a subset of
the bits can be done by simply shifting/masking value if needed.

Also use for_each_set_bit().

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Sean Paul <sean@poorly.run>
Link: http://patchwork.freedesktop.org/patch/msgid/20190923065814.4797-1-kraxel@redhat.com