drm/amd/display: change generic_reg_wait to void.

we were only checking the return value in one place, thus changing
generic_reg_wait from int to void and reading the register instead of
getting it from generic_reg_wait, when we need the return value.

Signed-off-by: Yongqiang Sun <yongqiang.sun@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>