media: stm32-dma2d: fix compile errors when W=1

remove unused functions to avoid compile errors when W=1.

>> drivers/media/platform/stm32/dma2d/dma2d-hw.c:29:20:
	error: unused function 'reg_set' [-Werror,-Wunused-function]
	static inline void reg_set(void __iomem *base, u32 reg, u32 mask)

>> drivers/media/platform/stm32/dma2d/dma2d-hw.c:34:20:
	error: unused function 'reg_clear' [-Werror,-Wunused-function]
	static inline void reg_clear(void __iomem *base, u32 reg, u32 mask)

Fixes: bff6e3e2f4c9 ("media: stm32-dma2d: STM32 DMA2D driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dillon Min <dillon.minfei@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>