drm/sprd: add Unisoc's drm display controller driver

Adds DPU(Display Processor Unit) support for the Unisoc's display subsystem.
It's support multi planes, scaler, rotation, PQ(Picture Quality) and more.

RFC v7:
  - Remove plane_update stuff, dpu handles all the HW update in crtc->atomic_flush

RFC v6:
  - Access registers via readl/writel
  - Checking for unsupported KMS properties (format, rotation, blend_mode, etc) on plane_check ops
  - Remove always true checks for dpu core ops

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Signed-off-by: Kevin Tang <kevin.tang@unisoc.com>

drivers/gpu/drm/sprd/Kconfig

@@ -3,6 +3,7 @@ config DRM_SPRD
 	depends on ARCH_SPRD || COMPILE_TEST
 	depends on DRM && OF
 	select DRM_KMS_HELPER
+	select VIDEOMODE_HELPERS
 	select DRM_GEM_CMA_HELPER
 	select DRM_KMS_CMA_HELPER
 	select DRM_MIPI_DSI

drivers/gpu/drm/sprd/Makefile

@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
-subdir-ccflags-y += -I$(srctree)/$(src)
+obj-y := sprd_drm.o \
+	sprd_dpu.o
+
+obj-y += dpu_r2p0.o
 
-obj-y := sprd_drm.o