drm/amd/display: Explicitly extend unsigned 16 bit to 64 bit

Coverity reports sign extention defects as below:

Suspicious implicit sign extension: mode->htotal with type u16 ... to
int (32 bits, signed), then sign-extended to type unsigned long
(64 bits, unsigned). If mode->htotal * mode->vtotal is greater than
0x7FFFFFFF, the upper bits of the result will all be 1.

Cast it to unsigned long to avoid possible overflow.

This fixes 4 SIGN_EXTENSION issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>