drm/amd/display: Fix possible use of uninitialized 'max_chunks_fbc_mode' in 'calculate_bandwidth()'

'max_chunks_fbc_mode' is only declared and assigned a value under a
specific condition in the following lines:

if (data->fbc_en[i] == 1) {
	max_chunks_fbc_mode = 128 - dmif_chunk_buff_margin;
}

If 'data->fbc_en[i]' is not equal to 1 for any i, max_chunks_fbc_mode
will not be initialized if it's used outside of this for loop.

Ensure that 'max_chunks_fbc_mode' is properly initialized before it's
used. Initialize it to a default value right after its declaration to
ensure that it gets a value assigned under all possible control flow
paths.

Thus fixing the below:
drivers/gpu/drm/amd/amdgpu/../display/dc/basics/dce_calcs.c:914 calculate_bandwidth() error: uninitialized symbol 'max_chunks_fbc_mode'.
drivers/gpu/drm/amd/amdgpu/../display/dc/basics/dce_calcs.c:917 calculate_bandwidth() error: uninitialized symbol 'max_chunks_fbc_mode'.

Fixes: 4562236b

 ("drm/amd/dc: Add dc display driver (v2)")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>