drm/amd/display: fix math_mod() using arg1 instead of arg2

drm/amd/display: fix math_mod() using arg1 instead of arg2

[Why]
math_mod() multiplied by arg1 instead of arg2, returning a wrong
result for any non-trivial modulo operation.

[How]
Replace arg1 with arg2 in the subtraction term to correctly
implement fmod(arg1, arg2).

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>