drm/amd/display: Remove unused clk_src variable

Remove the variable clk_src from the function dcn3_get_pix_clk_dividers.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_clock_source.c:1279:25: warning:
variable 'clk_src' set but not used [-Wunused-but-set-variable]
        struct dce110_clk_src *clk_src;
                               ^
1 warning generated.

Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>