drm/amd/display: Don't allow partial copy_from_user

There is no reason to allow for partial buffers from userspace in our
debugfs. In this particular case callers will zero out the wr_buf but if
callers in the future don't do that we might be looking at corrupt data.

Linus puts it better than I can in
https://lkml.org/lkml/2021/10/26/993Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_debugfs.c

@@ -78,12 +78,10 @@ static int parse_write_buffer_into_params(char *wr_buf, uint32_t wr_buf_size,
 
 	wr_buf_ptr = wr_buf;
 
-	r = copy_from_user(wr_buf_ptr, buf, wr_buf_size);
-
 	/* r is bytes not be copied */
-	if (r >= wr_buf_size) {
-		DRM_DEBUG_DRIVER("user data not be read\n");
-		return -EINVAL;
+	if (copy_from_user(wr_buf_ptr, buf, wr_buf_size)) {
+		DRM_DEBUG_DRIVER("user data could not be read successfully\n");
+		return -EFAULT;
 	}
 
 	/* check number of parameters. isspace could not differ space and \n */