drm/xe: Avoid 64-bit register reads

drm/xe: Avoid 64-bit register reads

Intel hardware officially only supports GTTMMADR register accesses of
32-bits or less (although 64-bit accesses to device memory and PTEs in
the GSM are fine).  Even though we do usually seem to get back
reasonable values when performing readq() operations on registers in
BAR0, we shouldn't rely on this violation of the spec working
consistently.  It's likely that even when we do get proper register
values back the hardware is internally satisfying the request via a
non-atomic sequence of two 32-bit reads, which can be problematic for
timestamps and counters if rollover of the lower bits is not considered.

Replace xe_mmio_read64() with xe_mmio_read64_2x32() that implements
64-bit register reads as two 32-bit reads and attempts to ensure that
the upper dword has stabilized to avoid problematic rollovers for
counter and timestamp registers.

v2:
 - Move function from xe_mmio.h to xe_mmio.c.  (Lucas)
 - Convert comment to kerneldoc and note that it shouldn't be used on
   registers where reads may trigger side effects.  (Lucas)

Bspec: 60027
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://lore.kernel.org/r/20230823003312.1356779-3-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>