drm/amd/display/dc/dce110/dce110_timing_generator: Remove unused variable 'value_crtc_vtotal'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_timing_generator.c: In function ‘dce110_timing_generator_tear_down_global_swap_lock’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_timing_generator.c:1354:12: warning: variable ‘value_crtc_vtotal’ set but not used [-Wunused-but-set-variable]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Aric Cyr <Aric.Cyr@amd.com>
Cc: Anthony Koo <Anthony.Koo@amd.com>
Cc: Tony Cheng <Tony.Cheng@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>