drm/i915: Don't store the max cursor width/height in the crtc

Those values are, global, only used in one function and already stored
in mode_config.cursor_{width,height}.

As a result, this initialization code has been moved from the
crtc_init() function to the global modeset_init() one.

I also renamed CURSOR_{WIDTH,HEIGHT} to MAX_CURSOR_{WIDTH,HEIGHT} to be
more accurate about what these value really are.

Cc: Sagar Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>