drm/amdgpu: Use RMW accessors for changing LNKCTL2

Convert open coded RMW accesses for LNKCTL2 to use
pcie_capability_clear_and_set_word() which makes its easier to
understand what the code tries to do.

LNKCTL2 is not really owned by any driver because it is a collection of
control bits that PCI core might need to touch. RMW accessors already
have support for proper locking for a selected set of registers
(LNKCTL2 is not yet among them but likely will be in the future) to
avoid losing concurrent updates.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>