drm/amd/display: fix compilation error due to shifting negative value

Currently compiling linux-next with allmodconfig triggers the following
error:

./drivers/gpu/drm/amd/amdgpu/../display/include/fixed31_32.h: In function ‘dc_fixpt_truncate’:
./drivers/gpu/drm/amd/amdgpu/../display/include/fixed31_32.h:528:22: error: left shift of negative value [-Werror=shift-negative-value]
  528 |  arg.value &= (~0LL) << (FIXED31_32_BITS_PER_FRACTIONAL_PART - frac_bits);
      |                      ^~

Use `unsigned long long` instead.

Signed-off-by: GONG, Ruiqi <gongruiqi@huaweicloud.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>