drm/radeon: delete unused PTE_* defines

They don't appear to be used anywhere... elsewhere uses R*_PTE_*.

master@linux:U:.% git grep PTE_ -- drivers/gpu/drm/radeon | grep -v _PTE_
master@linux:U:.%	(kyle@redacted:~/linux)

./arch/arm64/include/asm/pgtable.h:27:0: note: this is the location of the previous definition
 #define PTE_VALID  (_AT(pteval_t, 1) << 0)
 ^
In file included from drivers/gpu/drm/radeon/r600_cs.c:31:0:
drivers/gpu/drm/radeon/r600d.h:48:0: warning: "PTE_VALID" redefined [enabled by default]
 #define PTE_VALID    (1 << 0)
 ^
In file included from ./arch/arm64/include/asm/io.h:29:0,
                 from include/linux/clocksource.h:19,
                 from include/clocksource/arm_arch_timer.h:19,
                 from ./arch/arm64/include/asm/arch_timer.h:27,
                 from ./arch/arm64/include/asm/timex.h:19,
                 from include/linux/timex.h:65,
<snip>
                 from include/drm/drmP.h:51,
                 from drivers/gpu/drm/radeon/r600_cs.c:29:
./arch/arm64/include/asm/pgtable.h:27:0: note: this is the location of the previous definition
 #define PTE_VALID  (_AT(pteval_t, 1) << 0)
 ^

Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>