drm/amd/display: add some math functions for dcn_calc_math

Implement floor, ceil, and fabs

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>