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 <[e-mail]>
Signed-off-by: Melissa Wen <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/modules/color/color_gamma.c

···
 	struct pwl_float_data_ex *curve = NULL;
 	struct gamma_pixel *axis_x = NULL;
 	struct pixel_gamma_point *coeff = NULL;
-	enum dc_transfer_func_predefined tf = TRANSFER_FUNCTION_SRGB;
+	enum dc_transfer_func_predefined tf;
 	uint32_t i;
 	bool ret = false;
 
···
 	struct pwl_float_data_ex *rgb_regamma = NULL;
 	struct gamma_pixel *axis_x = NULL;
 	struct pixel_gamma_point *coeff = NULL;
-	enum dc_transfer_func_predefined tf = TRANSFER_FUNCTION_SRGB;
+	enum dc_transfer_func_predefined tf;
 	bool do_clamping = true;
 	bool ret = false;