drm/amd/display: Remove redundant NULL check in dcn10_set_input_transfer_func

This commit removes an unnecessary NULL check in the
`dcn10_set_input_transfer_func` function in the `dcn10_hwseq.c` file.
The variable `tf` is assigned the address of
`plane_state->in_transfer_func` unconditionally, so it can never be
`NULL`. Therefore, the check `if (tf == NULL)` is unnecessary and has
been removed.

Fixes the below smatch warning:
drivers/gpu/drm/amd/amdgpu/../display/dc/hwss/dcn10/dcn10_hwseq.c:1839 dcn10_set_input_transfer_func() warn: address of 'plane_state->in_transfer_func' is non-NULL

Fixes: 285a7054 ("drm/amd/display: Remove plane and stream pointers from dc scratch")
Cc: Wenjing Liu <wenjing.liu@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Alvin Lee <alvin.lee2@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>