The AD&D Player's Handbook mentions movement rate for dwarves under neither the discussion of dwarves, nor the discussion of movement. Yet the discussion of dwarves explicitly states (page 15):
The race of dwarves typically dwells in hilly or mountainous regions. For details of the race in general the reader is referred to ADVANCED DUNGEONS & DRAGONS, MONSTER MANUAL. As player characters, both dwarves and their cousins the "mountain dwarves" can be considered.
So, we refer to the Monster Manual (page 35). Their MOVE score is stated as 6". Regarding some of the other short PC races, their MOVE scores are: halflings - 9" (page 50), gnomes - 6" (page 46).
AD&D Second Edition clears possible confusions and ties a lot of loose ends while staying largely compatible with AD&D (as it has been designed that way). So we can also have a look at the AD&D 2e Player's Handbook for some more guidance. In the section on Movement (page 119), we are now given clearly the base movement rates for all the PC races: elves, half-elves and humans have 12, dwarves, gnomes and halflings have 6. Effects of encumbrance are described on page 79 as follows:
Light reduces the movement rate by ⅓ (round fractions down), Moderate reduces it by ½, Heavy reduces it by ⅔, and Severe lowers the movement rate to 1.
Finally, note that movement is measured in inches in AD&D; how these units convert to distance in various settings are described in the Movement section of the Player's Handbook.