ARM: 8578/1: mm: ensure pmd_present only checks the valid bit

In a subsequent patch, pmd_mknotpresent will clear the valid bit of the
pmd entry, resulting in a not-present entry from the hardware's
perspective. Unfortunately, pmd_present simply checks for a non-zero pmd
value and will therefore continue to return true even after a
pmd_mknotpresent operation. Since pmd_mknotpresent is only used for
managing huge entries, this is only an issue for the 3-level case.

This patch fixes the 3-level pmd_present implementation to take into
account the valid bit. For bisectability, the change is made before the
fix to pmd_mknotpresent.

[catalin.marinas@arm.com: comment update regarding pmd_mknotpresent patch]

Fixes: 8d962507 ("ARM: mm: Transparent huge page support for LPAE systems.")
Cc: <stable@vger.kernel.org> # 3.11+
Cc: Russell King <linux@armlinux.org.uk>
Cc: Steve Capper <Steve.Capper@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>