rpi: avoid vc_dispmanx_set_wl_buffer_in_use without EGL

rpi: avoid vc_dispmanx_set_wl_buffer_in_use without EGL

The symbol is needed only for the EGL buffer path. If --disable-egl is
given to ./configure, there is no need for it, so fix it to actually not
look for that symbol needlessly.

This should fix the runtime error:

	Failed to load module: .../rpi-backend.so: undefined symbol:
	vc_dispmanx_set_wl_buffer_in_use

when you use --disable-egl and do not have a recent enough
libraspberrypi package (/opt/vc, a.k.a userland.git) that would provide
vc_dispmanx_set_wl_buffer_in_use. Apparently no released version of
userland yet provides this.

The calls are organized into two helper functions to avoid a boolean
argument, and put the #ifdefs away from the main parts of the code.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Tomeu Vizoso <tomeu@tomeuvizoso.net>