drm/radeon: Fix ENOSYS with better fitting error codes in radeon_gem.c

Replace the error code from 'ENOSYS' to 'EOPNOTSUPP' for unimplemented
radeon_gem_pread_ioctl & radeon_gem_pwrite_ioctl

Fixes the following:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else.

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>