drm/xe: Cleanup page-related defines

Rename the following defines to lose the GEN* prefixes since they don't
make sense for xe:

GEN8_PTE_SHIFT		-> XE_PTE_SHIFT
GEN8_PAGE_SIZE		-> XE_PAGE_SIZE
GEN8_PTE_MASK		-> XE_PTE_MASK
GEN8_PDE_SHIFT		-> XE_PDE_SHIFT
GEN8_PDES		-> XE_PDES
GEN8_PDE_MASK		-> XE_PDE_MASK
GEN8_64K_PTE_SHIFT	-> XE_64K_PTE_SHIFT
GEN8_64K_PAGE_SIZE	-> XE_64K_PAGE_SIZE
GEN8_64K_PTE_MASK	-> XE_64K_PTE_MASK
GEN8_64K_PDE_MASK	-> XE_64K_PDE_MASK
GEN8_PDE_PS_2M		-> XE_PDE_PS_2M
GEN8_PDPE_PS_1G		-> XE_PDPE_PS_1G
GEN8_PDE_IPS_64K	-> XE_PDE_IPS_64K
GEN12_GGTT_PTE_LM	-> XE_GGTT_PTE_LM
GEN12_USM_PPGTT_PTE_AE	-> XE_USM_PPGTT_PTE_AE
GEN12_PPGTT_PTE_LM	-> XE_PPGTT_PTE_LM
GEN12_PDE_64K		-> XE_PDE_64K
GEN12_PTE_PS64		-> XE_PTE_PS64
GEN8_PAGE_PRESENT	-> XE_PAGE_PRESENT
GEN8_PAGE_RW		-> XE_PAGE_RW
PTE_READ_ONLY		-> XE_PTE_READ_ONLY

Keep an XE_ prefix to make sure we don't mix the defines for the CPU
(e.g. PAGE_SIZE) with the ones fro the GPU).

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>