Improve drmmode_copy_fb error handling
Improve drmmode_copy_fb error handling

 * correctly check mmap() failure value (MAP_FAILED, not NULL)
 * check all return values
 * print error messages with errno strings
 * unmap if cpu_prep fails

Signed-off-by: Daniel Kurtz <31914e832df1cd8e5daa417106c0c6f5297b27d7@chromium.org>

BUG=none
TEST=build and run manually

Change-Id: Ief58f822c57a4bc945b4a167a7f3c22c482de73f
Reviewed-on: https://chromium-review.googlesource.com/65849
Reviewed-by: Daniel Kurtz <31914e832df1cd8e5daa417106c0c6f5297b27d7@chromium.org>
Commit-Queue: Daniel Kurtz <31914e832df1cd8e5daa417106c0c6f5297b27d7@chromium.org>
Tested-by: Daniel Kurtz <31914e832df1cd8e5daa417106c0c6f5297b27d7@chromium.org>

markyzq/armsoc-rockchip,markyzq/armsoc-rockchip