ARM: 8655/1: improve NOMMU definition of pgprot_*()

The tegra DRM driver produces a harmless warning when built for NOMMU:

drivers/gpu/drm/tegra/gem.c: In function 'tegra_drm_mmap':
drivers/gpu/drm/tegra/gem.c:508:12: unused variable 'prot'

This is because pgprot_writecombine() on ARM returns a constant and
ignores its argument. The version in asm-generic doesn't have that
problem, so let's use that one instead. We don't actually care
about the value on NOMMU, and this is consistent with what some
other architectures do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>