x86: Avoid using hardcoded number of variable range MTRRs in mtrr_commit()

x86: Avoid using hardcoded number of variable range MTRRs in mtrr_commit()

Since commit 29d2d64ed55f ("x86: Add support for more than 8 MTRRs"),
the maximum number of variable range MTRRs was increased from 8 to 10.
On the BayTrail platform there are only 8 variable range MTRRs. In
mtrr_commit() it still uses MTRR_MAX_COUNT which caused a #GP during
VESA video driver probe. It should have been updated to use dynamically
probed number.

This fixes the boot failure seen on Intel Minnow Max board.

Fixes: 29d2d64ed55f ("x86: Add support for more than 8 MTRRs")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>