drm/amd/display: Enable building new display engine with KCOV enabled

The new display engine uses floating point math, which is not supported
by KCOV. Commit 9d1d02ff3678 ("drm/amd/display: Don't build DCN1 when kcov
is enabled") tried to work around the problem by disabling
CONFIG_DRM_AMD_DC_DCN if KCOV_INSTRUMENT_ALL and KCOV_ENABLE_COMPARISONS
are enabled. The result is that KCOV can not be enabled on systems which
require this display engine. A much simpler and less invasive solution is
to disable KCOV selectively when compiling the display enagine while
keeping it enabled for the rest of the kernel.

Fixes: 9d1d02ff3678 ("drm/amd/display: Don't build DCN1 when kcov is enabled")
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>