{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/cpu/x86/mp_init.c

@@ -247,11 +247,9 @@ static int save_bsp_msrs(char *start, int size)
 	int num_var_mtrrs;
 	struct saved_msr *msr_entry;
 	int i;
-	msr_t msr;
 
 	/* Determine number of MTRRs need to be saved. */
-	msr = rdmsr(MTRR_CAP_MSR);
-	num_var_mtrrs = msr.lo & 0xff;
+	num_var_mtrrs = get_var_mtrr_count();
 
 	/* 2 * num_var_mtrrs for base and mask. +1 for IA32_MTRR_DEF_TYPE. */
 	msr_count = 2 * num_var_mtrrs + NUM_FIXED_MTRRS + 1;