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

Which introduced in Link.

Link:
https://lore.kernel.org/lkml/mhng-3e4b97f6-8310-487e-9f9a-1d19d6e42a1e@palmer-ri-x1c9/
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

@@ -45,12 +45,7 @@ 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
+dml_ccflags := -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)([^v_]*)v?/\1\2/')
 endif
 
 ifdef CONFIG_CC_IS_GCC