drm/amd/display: Fix FIXED_VS retimer clock gen source override

[WHY]
For SQ128 pattern some vendor-specific overrides are required.
Previously a hardcoded clock gen source value was incorrectly programmed,
causing our override to retimer's clock source override to be ignored.
Due to some PHY issues on certain APU programs, we see failures on retimer
bypass ports extend to electrical testing downstream of PHY due to some host
clock jitter which the retimer follows.

[HOW]
Fix typo to use correct clock gen source override of 0xC4 rather than 0x4C.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>