drm/amd/display: remove some unneeded code

We assign "v_init = asic_blank_start;" a few lines earlier so there is
no need to do it again inside the if statements.  Also "v_init" is
unsigned so it can't be less than zero.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>