Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/radeon/radeon_legacy_tv.c: In function radeon_legacy_tv_mode_set:
drivers/gpu/drm/radeon/radeon_legacy_tv.c:538:24: warning: variable tv_pll_cntl1 set but not used [-Wunused-but-set-variable]

It is introduced by commit 4ce001ab ("drm/radeon/kms:
add initial radeon tv-out support."), but never used,
so remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>