drm/omap: dsi: Rework and remove a few unused variables

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

 drivers/gpu/drm/omapdrm/dss/dsi.c: In function ‘_dsi_print_reset_status’:
 drivers/gpu/drm/omapdrm/dss/dsi.c:1131:6: warning: variable ‘l’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/omapdrm/dss/dsi.c: In function ‘dsi_update’:
 drivers/gpu/drm/omapdrm/dss/dsi.c:3943:10: warning: variable ‘dh’ set but not used [-Wunused-but-set-variable]
 drivers/gpu/drm/omapdrm/dss/dsi.c:3943:6: warning: variable ‘dw’ set but not used [-Wunused-but-set-variable]

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105144517.1826692-9-lee.jones@linaro.org