drm/amdkfd: Fix gfx10 wave64 VGPR context restore

Copy/paste error, first 4 VGPRs are separated by 64 dwords (256 bytes).

Cc: Shaoyun Liu <shaoyun.liu@amd.com>
Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: shaoyunl <shaoyun.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>