drm/amd/display: unset initial value for tf since it's never used

In mod_color_calculate_{degamma/regamma}_params(), a tf variable is
initialized as TRANSFER_FUNCTION_SRGB but tf is only used after tf =
input->tf, therefore, better to just remove this initial value and avoid
misleading interpretations.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>