gpu: drm: dead code elimination

this set adds support for removal of gpu drm dead code.

patch3 is similar with patch 1:
`num` is a data of u8 type and ATOM_MAX_HW_I2C_READ == 255,

so there is a impossible condition '(num > 255) => (0-255 > 255)'.

Signed-off-by: Pan Zhang <zhangpan26@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>