drm/amd/display: dereference variable before checking for zero

[Why]
Driver incorrectly checks if pointer variable OutBpp is null instead of
if the value being pointed to is zero.

[How]
Dereference OutBpp before checking for a value of zero.

Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>