iommu/arm-smmu: Fix SMR mask extraction for bootloader programmed SMRs

When extracting the mask for an SMR that was programmed by the
bootloader, the SMR.valid field is also extracted and is treated
as part of the mask. This causes issues when removing an SMMU
master, as removing the master involves constructing an SMR
entry with the mask and SID without the valid bit set and writing
that value to the SMR. However, since the valid bit was part of
the mask, the SMR will still be valid.

Instead, remove the valid bit from the raw SMR value that is used
for extracting the SMR mask.

Change-Id: Iccada2c474b196200ae1b56eb9faa9cb026f5ef1
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>