radeon: Remove useless quirk for zx1/FireGL X1 combo introduced with fdo #7770

Removes useless quirk a7f465f73363fce409870f62173d518b1bc02ae6 introduced with
fdo #7770 as a failed attempt to minimize stability issues with hp zx1 chipset/
ATI FireGL X1 graphics adapter configuration
(see http://marc.info/?l=linux-ia64&m=140077543819871&w=2

 for details/reason)

Signed-off-by: Émeric MASCHINO <emeric.maschino@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>