Fix some surface refcounting issues in display

[ankith26]

In get_surface, a strong reference was being incref-ed before being returned (double incref issue)

In set_mode there was a decref followed by an incref, which is technically wrong (I suppose in practice it doesn't create an issue, but I fixed it anyways)

[damusss]

Alright now that I understand it the bugfix makes sense, so it LGTM :)

[MyreMylar]

In set_mode there was a decref followed by an incref, which is technically wrong (I suppose in practice it doesn't create an issue, but I fixed it anyways)

What about the return here:

pygame-ce/src_c/display.c

Lines 1333 to 1339 in f5f850a

	if (depth != 0 && PG_SURF_BitsPerPixel(surface->surf) != depth) {
	if (PyErr_WarnEx(PyExc_DeprecationWarning,
	"The depth argument is deprecated, and is ignored",
	1)) {
	return NULL;
	}
	}

Won't removing the decref cause a leak if it returns NULL?