drm/amd/display/dc/calcs/dce_calcs: Remove some large variables from the stack

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dce_calcs.c: In function ‘bw_calcs_init’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dce_calcs.c:2726:1: warning: the frame size of 1336 bytes is larger than 1024 bytes [-Wframe-larger-than=]

v2: squash in sizeof fix

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>