glx: decouple dri2.c and GLX, fixing Gallium EGL and d3d1x build

glx: decouple dri2.c and GLX, fixing Gallium EGL and d3d1x build

The Gallium EGL state tracker reuses dri2.c but not the GLX code.

Currently there is a bit of code in dri2.c that is incorrectly tied
to GLX: instead, make it call an helper that both GLX and Gallium EGL
implement, like dri2InvalidateBuffers.

This avoids a link error complaining that dri2GetGlxDrawableFromXDrawableId
is undefined.

Note that we might want to move the whole event translation elsewhere,
and probably stop using non-XCB DRI2 altogether, but this seems to be
the minimal fix.