drm: xilinx: Kconfig: Remove depend on VDMA

The Xilinx DRM doesn't directly depend on VDMA. Remove dependency on VDMA,
and add dependency on DMA_ENGINE instead. Users can select the DMA engine
device based on their design.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>