egl: use coherent variable names

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
`EGLDisplay`s, and `disp` otherwise.

Let's be consistent and use `dpy` for the Khronos type, and `disp`
for our struct.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Eric Anholt <eric@anholt.net>