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

 drivers/gpu/drm/mga/mga_dma.c:29: warning: Cannot understand  * file mga_dma.c
 drivers/gpu/drm/mga/mga_dma.c:455: warning: Function parameter or member 'dev' not described in 'mga_do_agp_dma_bootstrap'
 drivers/gpu/drm/mga/mga_dma.c:455: warning: Function parameter or member 'dma_bs' not described in 'mga_do_agp_dma_bootstrap'
 drivers/gpu/drm/mga/mga_dma.c:629: warning: Function parameter or member 'dev' not described in 'mga_do_pci_dma_bootstrap'
 drivers/gpu/drm/mga/mga_dma.c:629: warning: Function parameter or member 'dma_bs' not described in 'mga_do_pci_dma_bootstrap'
 drivers/gpu/drm/mga/mga_dma.c:1150: warning: Function parameter or member 'dev' not described in 'mga_driver_unload'
 drivers/gpu/drm/mga/mga_dma.c:1159: warning: Function parameter or member 'dev' not described in 'mga_driver_lastclose'

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: by <jhartmann@precisioninsight.com>
Cc: Faith <faith@valinux.com>
Cc: Jeff Hartmann <jhartmann@valinux.com>
Cc: Keith Whitwell <keith@tungstengraphics.com>
Cc: Gareth Hughes <gareth@valinux.com>
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/20201106214949.2042120-5-lee.jones@linaro.org