drm/radeon/radeon_display: Remove unused variable 'mod'

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

 drivers/gpu/drm/radeon/radeon_display.c: In function ‘radeon_div’:
 drivers/gpu/drm/radeon/radeon_display.c:1094:11: warning: variable ‘mod’ set but not used [-Wunused-but-set-variable]

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: Sumit Semwal <sumit.semwal@linaro.org>
Cc: report to <xorg-driver-ati@lists.x.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>