drm/amd/display: Fix dc dml march cause riscv build failed

We must carefully when remove the march substr.
otherwise it will cause build failed when CONFIG_RISCV_V

Log:
  CC [M]  drivers/gpu/drm/amd/amdgpu/../display/dc/dml/display_mode_lib.o
riscv64-linux-gnu-gcc: error: ‘-march=r64imafdc_zicsr_zifencei_zihintpause’: ISA string must begin with rv32 or rv64

Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Author: opsiff

drivers/gpu/drm/amd/display/dc/dml/Makefile

@@ -45,13 +45,8 @@ endif
 
 ifdef CONFIG_RISCV
 include $(srctree)/arch/riscv/Makefile.isa
-ifdef CONFIG_RISCV_ISA_V
-# Remove V from the ISA string, like in arch/riscv/Makefile, but keep F and D.
-dml_ccflags := -march=$(subst v,,$(riscv-march-y))
-else
 dml_ccflags := -march=$(riscv-march-y)
 endif
-endif
 
 ifdef CONFIG_CC_IS_GCC
 ifneq ($(call gcc-min-version, 70100),y)