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 <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/atombios_i2c.c

···
 			memcpy(&out, &buf[1], num);
 		args.lpI2CDataOut = cpu_to_le16(out);
 	} else {
-		if (num > ATOM_MAX_HW_I2C_READ) {
-			DRM_ERROR("hw i2c: tried to read too many bytes (%d vs 255)\n", num);
-			r = -EINVAL;
-			goto done;
-		}
 		args.ucRegIndex = 0;
 		args.lpI2CDataOut = 0;
 	}