The function is not that big, but it's also not used for anything
performance critical. Make it a normal function.

As a side effect, this apparently makes sparse smarter about what it's
doing, and gets rid of the warning:

./include/drm/drm_color_mgmt.h:53:28: warning: shift too big (4294967295) for type unsigned long
./include/drm/drm_color_mgmt.h:53:28: warning: cast truncates bits from constant value (8000000000000000 becomes 0)

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485164579-16250-1-git-send-email-jani.nikula@intel.com