Use msm8974 directory for msm8226

- The code in msm8x26 directory is oudated and 8974 works
  with no issues.
- Remove unneeded msm8x26 directory
- Add symlink from msm8226 to msm8974 so current macros that
  use display/$(TARGET_BOARD_PLATFORM) to find needed headers
  continue to work as expected.
Change-Id: I5fe4a86674c9f278a951a6ad7a89af6f93c491cb