drm/radeon: Remove the references of radeon_gem_ pread & pwrite ioctls

Removing the functions of pread & pwrite & IOCTL defines, as their
existence allows an authorized client to spam the system logs.

Fixes: db996e64

 ("drm/radeon: Fix ENOSYS with better fitting error codes in radeon_gem.c")
Suggested-by: Christian König <christian.koenig@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>