drm/amd/display: Fix 64-bit division for 32-bit builds

[Why]
32-bit builds break when doing 64-bit division directly.

[How]
Use the div_u64() function instead to perform the division.

Fixes: https://lists.freedesktop.org/archives/dri-devel/2018-December/201008.htmlSigned-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce80/dce80_timing_generator.c

@@ -94,7 +94,7 @@ static void program_pix_dur(struct timing_generator *tg, uint32_t pix_clk_100hz)
 	if (pix_clk_100hz == 0)
 		return;
 
-	pix_dur = 10000000000ull / pix_clk_100hz;
+	pix_dur = div_u64(10000000000ull, pix_clk_100hz);
 
 	set_reg_field_value(
 		value,