drm/via/via_dma: Remove set but unused variable 'agp_base'

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

 drivers/gpu/drm/via/via_dma.c: In function ‘via_cmdbuf_jump’:
 drivers/gpu/drm/via/via_dma.c:596:11: warning: variable ‘agp_base’ set but not used [-Wunused-but-set-variable]

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112190039.2785914-5-lee.jones@linaro.org