egl/dri2: move wayland header inclusion where applicable

egl/dri2: move wayland header inclusion where applicable

Instead of indirectly pulling the wayland headers everywhere, use
forward declarations and #include only as needed.

Should effectively fix build errors like the following:

make[5]: Entering directory
'/.../src/gallium/state_trackers/omx/tizonia'
   CC       h264dprc.lo
In file included from h264dprc.c:45:0:
.../src/egl/drivers/dri2/egl_dri2.h:47:10: fatal error:
wayland/wayland-egl/wayland-egl-backend.h: No such file or directory
  #include "wayland/wayland-egl/wayland-egl-backend.h"

Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@imgtec.com>
Tested-by: Andy Furniss <adf.lists@gmail.com>