{cpu/x86, drivers/amd}: Use get_var_mtrr_count() to get MTRR count

This patch replaces the implementation that is used to get the number of
variable MTRRs with `get_var_mtrr_count()` function.

BUG=b:225766934
TEST=Able to build and boot google/redrix board to ChromeOS.

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: I4751add9c45374e60b7a425df87d06f52e6fcb8c
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63219
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/drivers/amd/agesa/mtrr_fixme.c

@@ -14,8 +14,7 @@ static void set_range_uc(u32 base, u32 size)
 {
 	int i, max_var_mtrrs;
 	msr_t msr;
-	msr = rdmsr(MTRR_CAP_MSR);
-	max_var_mtrrs = msr.lo & MTRR_CAP_VCNT;
+	max_var_mtrrs = get_var_mtrr_count();
 
 	for (i = 0; i < max_var_mtrrs; i++) {
 		msr = rdmsr(MTRR_PHYS_MASK(i));