drm/amd/display: Make sure line size is not zero in DCN2 line buffer size calculations

[why]
'Divide by zero' error happens when line size happens to be zero.

[how]
The code that makes sure line size minimum value can be 1 was already
present in DCN1 part of the driver, this is mearly a port to DCN2.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>