drm/amd/display: Fix implicit narrowing conversion warnings

drm/amd/display: Fix implicit narrowing conversion warnings

[Why]
Multiple display source files contain implicit narrowing
conversions when assigning wider integer types (int, uint32_t)
to narrower fields (uint8_t, uint16_t) at hardware register,
protocol, and storage boundaries. These conversions are
intentional but undocumented, and accompanying runtime assertions
add noise without providing compile-time safety.

[How]
Add explicit casts at all intentional narrowing boundaries across
display source files. Use narrower loop variable types where loop
bounds guarantee safe range. Remove runtime assertions paired
with narrowing casts, inline single-use intermediate variables,
and revert block scopes and braces introduced solely to contain
those assertions.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Gaghik Khachatrian <gaghik.khachatrian@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>