drm/pl111: Remove reverse dependency on DRM_DUMB_VGA_DAC

drm/pl111: Remove reverse dependency on DRM_DUMB_VGA_DAC

DRM_DUMB_VGA_DAC is a user-visible symbol. Selecting it can cause unmet
direct dependencies such as this (on i386, randconfig):

	warning: (DRM_PL111) selects DRM_DUMB_VGA_DAC which has unmet direct dependencies (HAS_IOMEM && DRM && DRM_BRIDGE && OF)

This is because DRM_DUMB_VGA_DAC depends on OF while DRM_PL111 does not.
It does indirectly depend on OF via the ARM and ARM64 dependencies, but
since it can also be enabled under COMPILE_TEST, randconfig can find a
case where DRM_PL111 is selected without pulling in OF and not meeting
the dependency for DRM_DUMB_VGA_DAC.

Since select is "heavy handed", DRM_DUMB_VGA_DAC is going to be enabled
regardless of the above warning and causes the following build error:

	../drivers/gpu/drm/bridge/dumb-vga-dac.c: In function 'dumb_vga_probe':
	../drivers/gpu/drm/bridge/dumb-vga-dac.c:207:13: error: 'struct drm_bridge' has no member named 'of_node'
	  vga->bridge.of_node = pdev->dev.of_node;

See Documentation/kbuild/kconfig-language.txt, "reverse dependencies".

Fixes: 49f81d80ab84 ("drm/pl111: Support handling bridge timings")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Archit Taneja <archit@cradlewise.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220102903.27787-1-thierry.reding@gmail.com