gfx: imgv: fix parameter checking for exec cmd IOCTL

gfx: imgv: fix parameter checking for exec cmd IOCTL

As the comment in code suggests the IOCTL accepts only flags specifying
a GPU read or write operation, for all other flags we should return
-EINVAL. The call to psb_ttm_bo_check_placement that would seemingly
check this is actually a no-op in this case. Replace it with an actual
check. While at it, also clean up the preceeding check.

This is also needed by an upcoming patch getting rid of
psb_ttm_bo_check_placement.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>