drm/shmem: drop pgprot_decrypted()

Was added by commit 95cf9264

 ("x86, drm, fbdev: Do not specify
encrypted memory for video mappings"), then it was kept through various
changes.

While vram actually needs decrypted mappings this is not correct for
shmem gem objects which live in main memory not io memory, so remove the
call.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200228104723.18757-1-kraxel@redhat.com