drm/xe: Increase the XE_PL_TT watermark

The XE_PL_TT watermark was set to 50% of system memory.
The idea behind that was unclear since the net effect is that
TT memory will be evicted to TTM_PL_SYSTEM memory if that
watermark is exceeded, requiring PPGTT rebinds and dma
remapping. But there is no similar watermark for TTM_PL_1SYSTEM
memory.

The TTM functionality that tries to swap out system memory to
shmem objects if a 50% limit of total system memory is reached
is orthogonal to this, and with the shrinker added, it's no
longer in effect.

Replace the 50% TTM_PL_TT limit with a 100% limit, in effect
allowing all graphics memory to be bound to the device unless it
has been swapped out by the shrinker.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/intel-xe/20250305092220.123405-8-thomas.hellstrom@linux.intel.com