drm/amd/display: Minor refactor of DP PHY test automation

[Why]
Improve readability and maintainability of code.

[How]
Refactor test pattern size calculation out of function
call parameter and store value in variable.

Signed-off-by: George Shen <george.shen@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>