drm/i915: move check of intel_crtc_cursor_set_obj() out

Move check inside intel_crtc_cursor_set_obj() to
intel_check_cursor_plane(), we only use it there so move them out to
make the merge of intel_crtc_cursor_set_obj() into
intel_check_cursor_plane() easier.

This is another step toward the atomic modesetting support and unification
of plane operations such pin/unpin of fb objects on i915.

v2: take Ville's comment: move crtc_{w,h} assignment a bit down in the
code
v3: take Ville's comment: kept only the restructuring changes, the rest of
the code was moved to a separated patch since it is a bug fix (we weren't
checking sizes when the fb was the same)

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
[danvet: Fixup commit message mixup.]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>