drm/amd/display: Fix potential integer overflow

Fix potential integer overflow by casting actual_calculated_clock_100hz
to u64, in order to give the compiler complete information about the
proper arithmetic to use.

Notice that such variable is used in a context that expects
an expression of type u64 (64 bits, unsigned) and the following
expression is currently being evaluated using 32-bit arithmetic:

actual_calculated_clock_100hz * post_divider

Fixes: 7a03fdf6

 ("drm/amd/display: fix 64bit division issue on 32bit OS")
Addresses-Coverity-ID: 1501691 ("Unintentional integer overflow")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>