drm/amd/display: remove unintended executable mode

Besides the intended change, commit 4cc1178e ("drm/amdgpu: replace DRM
prefix with PCI device info for gfx/mmhub") also set the source files
mmhub_v1_0.c and gfx_v9_4.c to be executable, i.e., changed fromold mode
644 to new mode 755.

Commit 241b2ec9 ("drm/amd/display: Add dcn30 Headers (v2)") added the
four header files {dpcs,dcn}_3_0_0_{offset,sh_mask}.h as executable, i.e.,
mode 755.

Set to the usual modes for source and headers files and clean up those
mistakes. No functional change.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>