drm/omap: fix crash if there's no video PLL

Commit 8a7eda76 ("drm: omapdrm: dispc:
Pass DISPC pointer to remaining dispc API functions") made dpi.c use
ctx->pll even when there's no PLL, causing a crash at modeset on AM4
EVM, and presumably all OMAP2/3 boards.

Fix this by having struct dpi_data pointer in the ctx instead, giving
access to dispc without going through the pll.

Fixes: 8a7eda76

 ("drm: omapdrm: dispc: Pass DISPC pointer to remaining dispc API functions")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Keerthy <j-keerthy@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405065537.29818-1-tomi.valkeinen@ti.com