drm/amd/display: Fix timer_per_pixel unit error

[why]
The units of the time_per_pixel variable were incorrect, this had to be
changed for the code to properly function.

[how]
The change was very straightforward, only required one line of code to
be changed where the calculation was done.

Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Oliver Logush <oliver.logush@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>