drm/amd/display: Fix a test CalculatePrefetchSchedule()

It is likely Height was expected here, instead of Width.

Test the correct variable.

Fixes: 17529ea2 ("drm/amd/display: Optimizations for DML math")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>