drm: using mul_u32_u32() requires linux/math64.h

Some pending include file cleanups produced this error:

In file included from include/linux/kernel.h:27,
                 from drivers/gpu/ipu-v3/ipu-dp.c:7:
include/drm/drm_color_mgmt.h: In function 'drm_color_lut_extract':
include/drm/drm_color_mgmt.h:45:46: error: implicit declaration of function 'mul_u32_u32' [-Werror=implicit-function-declaration]
   45 |                 return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(user_input, (1 << bit_precision) - 1),
      |                                              ^~~~~~~~~~~

Fixes: c6fbb6bc ("drm: Fix color LUT rounding")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231219145734.13e40e1e@canb.auug.org.au