egl: use coherent variable names
EGLDisplay variables (the opaque Khronos type) have mostly been
consistently called dpy, as this is the name used in the Khronos
specs.
However, _EGLDisplay variables (our internal struct) have been
randomly called dpy when there was no local variable clash with
EGLDisplays, and disp otherwise.
Let's be consistent and use dpy for the Khronos type, and disp
for our struct.
Cc: @evelikov